I believe I've encountered this, or a similar issue, a few times after leaving a session unattended for 10 or so minutes. It doesn't appear to happen repeatably for me, but I haven't troubleshooted thoroughly.
When I've noticed it, it's been with large-ish (several hundred tracks) projects. No midi tracks are routed to external hardware, but there are plenty of kontakt and Vienna Ensemble Pro instances. The project appears to perform just fine with plenty of headroom otherwise.
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