The property of BP (derived from C++) class "doesn't" change after assigning to specific object

Oh you are using a forward declaration. Yeah that sounds a good reason for Unreal to be fudged up. Well, did you tried to actually include your header if it’s possible ?
If not, try to use another class or even create a “testclass” to see if including the header fixes the issue.

If the forward declaration is the issue, you could submit a bug report :wink: