The symptom you have described sounds like a race condition - a situation where the call flow branches into two different paths (staying in the queue vs. going to voicemail). Normally, the branch that would stay in queue would be terminated. I suspect that there is likely some customization to the queue (or possibly even a base handler) that is causing this issue.
If you are familiar with using SnapShot to analyze IC logs, then I recommend looking at the timeline view for an IP log of that call to see where this happens. If not, I recommend working with your partner to identify the issue.