I’ve encountered the same problem with version 5.0.3-20979098
I noticed some of the custom BTTask/BTService did not load into UE when opening a project, causing the references to “not be found” in the behaviour tree.
I deleted the /binaries and /intermediate folder in the project folder then right-click on project file → generate visual studio project files. Then double-click on project file and rebuild the binaries when prompted. This seemed to fix the problem for me (all classes properly loaded and found in BTtree) when reopening project after closing. IE the class naming was not involved for me.