This bug is somehow related to the bug #472.
The console "plug" command plugs a device to a port without unplugging one that is already there.
That is mechanically impossible on the real MSX computer. The port will always stay idle for some time while one device is disconnected and a new one is connected. The lack of this disconnection step causes the autodetection algorithms to fail.
To reproduce the problem, run the same JOYTEST.COM utility that is attached to the bug #442, and try the following steps bellow:
1) Run Joytest v2.2 on MSX-DOS
2) open the openMSX console
3) type "plug joyporta mouse"
4) Move the mouse for some seconds to check that it's working, then leave it idle.
5) type "plug joyporta joymega"
The Joytest will fail to detect that the mouse was changed to a megadrive joypad and will keep showing "MSX mouse" as the device connected to the port. On a real MSX, it will always detect this change properly because the mechanical requirement of disconnecting/connecting devices.