Is it really a bug? It wouldn’t come to my mind to rely on FMOD to perfectly detect the moment an event (the nested loop) finishes (by lack of content) and perform a seamless transition to the next entry of a parent multi-instrument. I also thought going from one entry to another in a multi-instrument isn’t totally instantaneous (hence no possible seamless transition), but maybe I’m wrong on that point.
I think the more reliable way to resolve the problem here is to put the multi in a nested event (it also requires to remove the loop regions on each multi component), that way:
Even if the bug is resolved, this setup has the advantage to allow to have a slightly longer loop than 8 bars: the loop itself + a reverb tail. The reverb tail would then be blended to the next multi entry, helping for a smooth transition, without loosing the tempo synchronization.
However, note that your parent event relies on the tempo to synchronize your stingers, but the nested loops are asynchronous. This could theoretically cause some desynchronizations if the event is played a long time; a high event “priority” should be a good thing.