As part of [Link Removed], it was changed so soft actor references cannot be set in the editor for structure defaults. This is desired because the serialization behavior for defaults is complicated and error prone so could corrupt things.
However, this accidentally caused them to be uneditable in data tables that are instances of this struct, because they are accidentally being detected as "templates" when data tables are not templated data. This is a regression from 4.25, and is a UI only change because you can still import them from CSV/JSON properly
There's no existing public thread on this issue, so head over to Questions & Answers just mention UE-108222 in the post.