Max_length messages from error_messages do not override default message of existing MaxLengthValidators

RegisterForm is based on the CustomUser model and has max_length error messages of first_name and last_name set. realestate\auth2\forms.py:

class RegisterForm(UserCreationForm):
    class Meta:
        model = CustomUser
        fields = ["email", "phone", "first_name", "last_name", "birth_date"]
        error_messages = {
            "email": {"invalid": _("email.email"), "max_length": _("field.max.string").format(field=__("email"), max=255)},
            "first_name": {"max_length": _("field.max.string").format(field=__("first_name"), max=255)}, # it should override default message of existing MaxLengthValidator
            "last_name": {"max_length": _("field.max.string").format(field=__("last_name"), max=255)}, # it should override default message of existing MaxLengthValidator
        }
        widgets = {"birth_date": forms.DateInput(attrs={"type": "date"})}

    def __init__(self, *args, **kwargs):
        super().__init__(*args, **kwargs)

        # ...

        # 2 Input validation & sanitization
        for field in self.fields:
            if field != "password2":
                self.fields[field].error_messages["required"] = _("field.required").format(field=_(field))

        self.fields["password1"].validators.append(PasswordValidator())
        self.fields["password2"].required = False
        self.fields["phone"].validators.append(PhoneValidator(_("phone.regex")))
        self.fields["first_name"].validators.append(AlphaValidator(_("field.alpha").format(field=__("first_name"))))
        self.fields["last_name"].validators.append(AlphaValidator(_("field.alpha").format(field=__("last_name"))))
        self.fields["birth_date"].validators.append(AdultValidator(_("birth_date.adult")))

        # Printing form fields
        for field in self.fields:
            print({
                "field": field,
                "validators": self.fields[field].validators,
                "error_messages": self.fields[field].error_messages
            })

        print()

CustomUser has CharFields with specified maximum lengths: realestate\users\models.py:

class CustomUser(AbstractUser):
    username = None
    email = models.EmailField(max_length=255, unique=True)
    phone = models.CharField(max_length=15, unique=True)
    first_name = models.CharField(max_length=255)
    last_name = models.CharField(max_length=255)
    birth_date = models.DateField()
    created_at = models.DateTimeField(auto_now_add=True)
    updated_at = models.DateTimeField(auto_now=True)

The template (excerpt):

<div class="col-12">
            <div class="input-group">
              <div class="form-floating">
                {{ form.first_name }}
                <label for="first_name">Ime</label>
              </div>

              <div class="form-floating">
                {{ form.last_name }}
                <label for="last_name">Prezime</label>
              </div>
            </div>

            <div class="row row-cols-2 g-0">
              {% if form.first_name.errors or form.last_name.errors %}
                <span class="col form-text text-danger">{{ form.first_name.errors.0 }}</span>
              {% endif %}
              
              {% if form.first_name.errors or form.last_name.errors %}
                <span class="col form-text text-danger">{{ form.last_name.errors.0 }}</span>
              {% endif %}
            </div>
          </div>

I printed the form fields and its validators and error messages to console.

{'field': 'first_name', 'validators': [<django.core.validators.MaxLengthValidator object at 0x0000029123170D70>, <django.core.validators.ProhibitNullCharactersValidator object at 0x00000291230B39D0>, <globals.validators.AlphaValidator object at 0x00000291236F96D0>], 'error_messages': {'required': 'Polje ime je obavezno.', 'max_length': 'Ime ne sme imati više od 255 karaktera.'}}
{'field': 'last_name', 'validators': [<django.core.validators.MaxLengthValidator object at 0x0000029123170DD0>, <django.core.validators.ProhibitNullCharactersValidator object at 0x00000291230B3A70>, <globals.validators.AlphaValidator object at 0x000002912381E780>], 'error_messages': {'required': 'Polje prezime je obavezno.', 'max_length': 'Prezime ne sme imati više od 255 karaktera.'}}

If these fields already have MaxLengthValidators, their messages should be overridden with the ones from error_messages. When I submit the form with first_name longer than 255, the new max_length error message does not show. I entered SlAQgECJZYdNBxpdBcWTVwIteYLcavXcBdwNJUSaQDcoJhKnFbmfZGtoVMuErstpFeExSfcFxjxiuFvParFfWTtdSNXtvjwwCJhkDCmqKnFQbGixeesvvPenAHtQxgUkSdkBuUgasThLyoBWPjnwXqLoHOhrkGpdcDuekxFPCDseGHFfvZpuUvrImKqNvECnabQZkpudPyxpRwjSREYbmsclBtRTFKlLZBYSNCprVVSgkIokzFJkEMnQizaKyER, which has 256 characters, and it somehow passed.

This happens unless I append additional MaxLengthValidator to the field's validators in the constructor

self.fields["first_name"].validators.extend([AlphaValidator(_("field.alpha").format(field=__("first_name"))), MaxLengthValidator(255)])
self.fields["last_name"].validators.extend([AlphaValidator(_("field.alpha").format(field=__("last_name"))), MaxLengthValidator(255)])

This works and the max_length error message from error_messages shows, but then the field has two MaxLengthValidators.

{'field': 'first_name', 'validators': [<django.core.validators.MaxLengthValidator object at 0x00000275F08A0D70>, <django.core.validators.ProhibitNullCharactersValidator object at 0x00000275F07E79D0>, <globals.validators.AlphaValidator object at 0x00000275F09C8050>, <django.core.validators.MaxLengthValidator object at 0x00000275F0F405F0>], 'error_messages': {'required': 'Polje ime je obavezno.', 'max_length': 'Ime ne sme imati više od 255 karaktera.'}}
{'field': 'last_name', 'validators': [<django.core.validators.MaxLengthValidator object at 0x00000275F08A0DD0>, <django.core.validators.ProhibitNullCharactersValidator object at 0x00000275F07E7A70>, <globals.validators.AlphaValidator object at 0x00000275F0DE1310>, <django.core.validators.MaxLengthValidator object at 0x00000275F0F40530>], 'error_messages': {'required': 'Polje prezime je obavezno.', 'max_length': 'Prezime ne sme imati više od 255 karaktera.'}}

What's stopping me from overriding error messages of existing MaxLengthValidators?

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