I’ve spent some time today investigating this further and I believe the fundamental problem is still an issue of references to FMOD assets being deserialized before the FMODStudio module has initialized and created the assets. Your workaround makes sense. By using a soft reference to the Event / Bus / VCA the engine won’t try to load the FMOD asset when it encounters the reference, and as long as the asset has been created before trying to execute the blueprint everything will work as intended.
Referencing FMOD assets directly in blueprint nodes as you were initially definitely works as long as the FMODStudio module startup has created the assets before the blueprint is loaded.
Looking at your log again, it appears that the UE4 streaming system is precaching the assets from your PAK file before the FMODStudio module is initialized, thus triggering the “Loading blueprints before plugin load” issue which Cameron pointed out previously. We have never encountered this issue arising this way before and so far I’ve been unable to reproduce it directly so I’m just going on your log for the time being. I’ll continue to work on reproducing the issue and if we can figure it out we can review our options for fixing it.
In the meantime the workaround you’ve found is probably the best solution. One alternative you might want to try is disabling the UE4 async loading thread (in Project Settings / Engine / Streaming). I’m not certain but I suspect that would also prevent the issue occurring.