There is an issue in the engine caused by UE-295700. Due to the FMODCallbackHandler.h file being in the same directory as the folder named FMOD,and that folder being added to the PublicIncludePathsin the Build.cs, when the UHT attempts to generate a .cpp for the header it chops the front of the filepath off thinking that it’s a relative directory path.
Even if our naming convention isn’t ideal and may not be expected, the fact that they are just string comparing the front of the path without checking if it’s a directory doesn’t seem great.
We have a workaround for this which will go out in the next release. In the next major FMOD version we will likely remove the old file completely.