Getting `ManagementForm data is missing or has been tampered with` error on every save

[rjmackay]

Using v0.5.3 with Wagtail 2.9.0 and Django 3.0.7.
I'm trying to use CondensedInlinePanel in the ModelAdmin, but while it renders beautifully, it won't save.

Here's the panel config:

        CondensedInlinePanel(
            "menu_items",
            label="Menu Items",
            card_header_from_field="title",
            panels=[
                FieldPanel("day_of_week"),
                FieldPanel("title"),
                FieldPanel("description"),
                FieldPanel("portion_per_order"),
                ImageChooserPanel("image"),
                FieldPanel("price"),
                FieldPanel("allergens"),
                FieldPanel("menu_category"),
                FieldPanel("order_limit"),
                FieldPanel("highlight"),
                FieldPanel("include_in_portion_count"),
            ],
        ),

[rjmackay]

It's possible this is related to an interaction with ModelCluster and/or taggit. The traceback seemed to be getting stuck on my allergens field, which is a tag field.

If I comment out that field and the MenuItemAllergen model (ie. the through model for allergens), then the error goes away.
I'm not sure I can follow enough of the internals of this app to debug it in detail though.