QuerySet.none() on combined queries returns all results.

Description

I came across this issue on Stack Overflow. I'm not 100% sure it's a bug, but it does seem strange. With this code (excuse the bizarre example filtering):
class Publication(models.Model):
pass
class Article(models.Model):
publications = models.ManyToManyField(to=Publication, blank=True, null=True)
class ArticleForm(forms.ModelForm):
publications = forms.ModelMultipleChoiceField(
Publication.objects.filter(id__lt=2) | Publication.objects.filter(id__gt=5),
required=False,
)
class Meta:
model = Article
fields = ["publications"]
class ArticleAdmin(admin.ModelAdmin):
form = ArticleForm
This works well. However, changing the ModelMultipleChoiceField queryset to use union() breaks things.
publications = forms.ModelMultipleChoiceField(
Publication.objects.filter(id__lt=2).union(
Publication.objects.filter(id__gt=5)
),
required=False,
)
The form correctly shows only the matching objects. However, if you submit this form while empty (i.e. you didn't select any publications), ALL objects matching the queryset will be added. Using the OR query, NO objects are added, as I'd expect.