Conditional formatting breaks custom layouts

I raised a Support ticket with Microsoft for this but was told to post here instead...
[Case #:24222483]

I have used the standard layout formatting to add sections to the BODY of the data entry form for a SharePoint list.

https://docs.microsoft.com/en-us/sharepoint/dev/declarative-customization/list-form-configuration#configure-custom-body-with-one-or-more-sections

But then when I configure a field to be hidden based on another field's value, the BODY formatting I just added stops working.

https://docs.microsoft.com/en-us/sharepoint/dev/declarative-customization/list-form-conditional-show-hide#get-started-with-conditional-formulas

On close inspection it seems that the JSON string is truncated and is therefore invalid. That's why the BODY formatting stops working.

Why does the formatting JSON for forms of lists and libraries get truncated when configuring column display settings?

This is a modern team site that is not connected to an Office 365 Group and there are no customisations loaded.