Thanks for the suggestions. Updating firmware and menu to latest ones listed in the dedicated thread (r1169 and 20191122) resolved the issue. Revision / build numbers shown in the menu did not change, though.
My Multi-MVS has corrupted graphics and garbled sound in menu (picture below), but games themselves seem to be working without issues. Any idea what could be
wrong? Tested so far only on MV-1C board.