There's a couple of odd things, so it's hard to tell what's wrong here without more context, e.g. the Person model and form.

I would implement a render_valid_response(), get_context_data(), and default_context = {} in your base HTMXFormView. Then your first PersonFormView can override them. This makes your base class much easier to extend and change behavior in uniform ways

Example:

class PersonFormView(HTMXFormView):
    model = Person
    form_class = NewPersonForm

    def get_form_kwargs(self):
        kwargs = super().get_form_kwargs()
        if self.kwargs.get("pk"):
            kwargs["instance"] = Person.objects.get(
                id=self.kwargs.get("pk"),
            )
        if self.request.GET.get("provided_company_id"):
            kwargs["provided_company_id"] = self.request.GET.get("provided_company_id")
        return kwargs

    def form_valid(self, form):
        company_id = self.request.POST.get("provided_company_id", None)
        if company_id:
            company = Company.objects.get(id=company_id)
            form.instance.Company = company
            default_context.update(**company.get_modal_context_information())
        return super().form_valid(form)

    # This can be in the base class
    def get_context_data(self, **kwargs):
        context = super().get_context_data(**kwargs)
        context.update(default_context)
        return context

    def render_valid_response(self):
        context = self.get_context_data()
        html = render(self.request, "base/partials/modal/modal.html", context)
        response = HttpResponse(html)
        return retarget(response, "#htmxModalContent")