The full description is in this FreeBSD bug report: https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=213919 but I think this is very likely the xorg bug. I observe this with USB mouse. Plugging/unplugging doesn't help. Restarting xorg helps, which suggests that the problem is in xorg, because it reopens the device every time it is plugged again and still has the problem.
Makes the system unusable.
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