duplicating translation in admin.save_model shows blank translations afterwards

[gfavre]

Hi,
I noticed an issue in hvad admin (v. 1.8.0)
General usage scenario for me is:

1. we serve documents in 4 languages
2. better serving a copy of one language into another than nothing.
3. I need to show all translations with django rest framework (works perfectly)

(Simplified) model:

from django.db import models

from hvad.models import TranslatableModel, TranslatedFields


class BindingDocument(TranslatableModel):
    translations = TranslatedFields(
        document=models.FileField(),
        label=models.CharField(_("Document label"), max_length=255),
    )

admin:

from django.contrib import admin
from django.conf import settings

from hvad.admin import TranslatableAdmin

from .models import BindingDocument


@admin.register(BindingDocument)
class BindingDocumentAdmin(TranslatableAdmin):
    list_display = ('all_translations',)
    
    def get_queryset(self, request):
        return BindingDocument.objects.prefetch_related('translations')

    def save_model(self, request, obj, form, change):
        super().save_model(request, obj, form, change)
        document = obj.document
        label = obj.label

        for (language_code, _) in settings.LANGUAGES:
            if obj.translations.filter(language_code=language_code).exists():
                continue
            obj.translate(language_code)
            obj.document = document
            obj.label = label
            obj.save()

Database is filled correctly (i.e. one translation per language is created, all contain same data), rest framework (using TranslationsMixin) also works perfectly.
Admin shows all languages in list view, as expected. However when going to any detail page other than the language in which translation was created, the result is blank. When entering content in blank fields, I get a duplicate key value violates unique constraint "products_bindingdocument_language_code_master_id_add98726_uniqexception.

My guess is that it has something to do with cache internals. Looks like a bug to me, perhaps it has a workaround?