This is not a regression. Tested in //UE5/Release-5.0 CL20979098
Mass Entity Config not displaying or accepting drag and dropped State Tree assets. The only clear way to assign a State Tree is by creating a State Tree from the drop down options, which will also no visible if you clear the assignment.
Previous created State Trees are visible and can be selected. Dragging and dropping a State Tree would work or more information about why it can't be used.
Previous State Tress are not listened, drag and drop turns it red but no other information is given.
There's no existing public thread on this issue, so head over to Questions & Answers just mention UE-170920 in the post.