Hi, this is somewhat related to this issue (Structured Text Error), which still exists and is a pain in the butt.
I have an existing slice within a custom type with a handful of various field types in the repeatable section of the slice. When I added a key text field to this existing slice, one of the Rich Text Fields, which was null by default before, becomes populated with an empty string. However, when I delete the repeatable section and recreate it as it was before, the Rich Text Field remains null.
Why does this happen? This is very frustrating because it messes with frontend validation logic and it makes me nervous about editing custom types, which is the whole point of Prismic!