When the default value of an Instanced variable defined in C++ is changed, it is not reflected in the default value of an Actor placed on a Level.
In the reproduction procedure, when the default value of the Instanced variable is changed from Blueprint while LevelB is open, the change is not reflected in the default value of the Actor on LevelA.
On the other hand, if the default values are changed from Blueprint while LevelA is open, they are reflected immediately. This may cause problems when changing the default values.
ContextObjects=MyContextObjectA in Step 7
ContextObjects=MyContextObjectB (Blueprint's default value) is changed.
There's no existing public thread on this issue, so head over to Questions & Answers just mention UE-164500 in the post.