I created a new C++ blank project i added a new class with a new MyInt variable and i hot reloaded. Once that was done i dropped an instance and set the value of MyInt to 10. After that i create a new MyInt2 variable and hot reloaded and i did not lose the value of MyInt on the instance. I then created a blueprint that derives from the class and adding a third variable MyInt3 broke the instance once it hot reloaded. Whether i saved the bp or the map before did not change the outcome. So for the tests that i did, this issue appears to only happen on blueprints classes, and not with wih the c++ classes. When you hot reload he resets the blueprint default values to the c++ default values and with that all of the instances that derive from the blueprint also get their values reset.