We've been experiencing this issue, and I found a culprit that fixed our case today.
I'll explain what I did, and hopefully, it will help anyone who may have encountered it. I believe this to be a bug.
We recently enabled spatial audio on our project and have been experiencing this message excessively: "3D audio object limit exceeded; object xxxxx instigated by voice/bus will be mixed."
Upon thorough investigation using the Audio Object Profiler while profiling our game, we discovered an excessive number of audio objects labeled as 'xxxx' (No voice data).' Further inspection revealed that one of our events, which played a looping random container attached to a moving actor, was the source of the issue.
Everytime a sound would be played in the looping container, it would seem to immediately get converted to this "No voice data" audio object, which is never removed.
We would then very quickly exceed the total number of allowed audio objects and all our new audio objects become mixed in the main mix.
Looking at the routing of busses, we could see that the random container was routed to a newly added audio object bus which was routed to a processing bus.
Moving the audio object bus out of the processing bus, and into an existing mixing bus, fixed this issue.
For context, the processing bus had the Delay plugin, with Feedback enabled and set to 97, Process LFE On, and a State that would set the Wet/Dry Mix to -100 under normal circumstances, and 100 when a condition was met in our game.
So it's possible that the delay was causing the audio object to persist even though the Delay was, audibly, off