I'm on the latest release (Version 12.0.2509.20409) and I've noticed a bug in the Layout Designer.

I have a Rich Text field inside a collection.  I can mark the setting for "Previous data or new lookup data will be read-only, but user-added sets will be editable" on the field and it will save and store properly.  But the very next time I edit the form, the field becomes unchecked, and if I don't resolve it before saving again, the field is no longer read-only the next time a user utilizes the form.

I was really struggling to understand why this was happening at first, because I wasn't manually unchecking it, but it would work when I was testing, and then stop working later.  Eventually I realized that it isn't populating as checked when you re-open the form on the Layout page.

I don't think it matters to this behavior, but I have the field inside a section inside the collection (I wanted multiple fields to be left of the Rich Text field, so I used two sections side by side, to facilitate that, that's also why I used a collection instead of a table).