It’s unlikely that the issue is caused by the different engine version unless there is a bug in 5.4.4. You could check the bug tracker but probably it’s not the case since we are talking about a core engine functionality of calling C++ functions in blueprints.
The _API bit is there so that the code is exposed to other modules. I remember having some issues that were caused by it being missing. If the class is a part of a plugin that has a single module, it should have the form of <PLUGIN_NAME>_API. You can type it yourself based on the name of the module the class resides in or have it generated if your IDE supports it. I have experience with Rider where you could auto-generate plugins/modules/classes and it would add it accordingly.
Rather than copying the classes, you could try to create a blank project and only make the scene component. If you get it to work, you can check what’s different between the new project and the old one. It is unlikely that copying the classes is going to work unless you messed up your build files, etc.