For TMap variables of type struct created in blueprint, if change the default value in child blueprint and compile, it will restore to the default value of the parent actor.
This problem does not occur when a C++ struct is used for the TMap variable.
1. Open the attached sample project in editor.
2. Open "BP_Child" blueprint.
3. Set "false" in bool property. [Struct->Key(S_Struct)->MemberVar_0]
4. Compile blueprint.
Then, the bool value will restore to the default value (true).
There's no existing public thread on this issue, so head over to Questions & Answers just mention UE-124976 in the post.