I found the cause of this problem.
Due to the bug in Xorg it unloads the keyboard driver when it gets
signal from hald that keyboard is disconnected, even though there is
still another keyboard that is hooked up to kbdmux.
Looks like Xorg code assumes that there can only be one keyboard and
isn't aware of kbdmux.
This was when hald is enabled (by default), and ServerFlags section in
xorg.conf looked like this:
Option "AllowEmptyInput" "yes"
When I changed this section to this:
Option "AutoAddDevices" "False"
keyboard doesn't get affected, but mouse disappears for good when
unplugged and plugged back, since (I guess) Xorg ignores ums0 device errors.
I filed the following PRs with Xorg:
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