If I understand correctly RepNotify function defined in C++ will not get called on the Server (Dedicater or Listen) and will only be called on clients.
This is the opposite of BP functionality, in the case of blueprints it gets called on the Server too (judging from my testing).
The only resource I could find on RepNotify is the following:
https://docs.unrealengine.com/4.27/en-US/Resources/ContentExamples/Networking/1_4/
This states the BP functionality, but it is pretty confusing trying to convert BPs to C++. Would be nice if Epic clarified it in the docs or unified it (but that’s unlikely as that could break already existing games).