I received the MarcMooga's board today. As I expected the board is fine although I was able to reproduce the problem with missing note off messages. The pointer was the edrumor's problem.
After I intentionally created a MIDI loop in MIDI-OX, hitting a pad a few times and than breaking the loop (by either disabling MIDI Out or correcting View->Port Routings in MIDI-OX) I saw the same behaviour with missing note off messages. The only way to restore the proper MegaDrum operation after that is to power cycle MegaDrum (disconnect/reconnect a USB cable). After that all MIDI messages are sent by MegaDrum properly.
Obviously, creating a MIDI loop with MIDI data in the loop, even temporary, knocks off MegaDrum internal memory buffers and its behaviour after this is unpredictable.
I intend to add a new option to MegaDrum configuration, Enable MIDI Thru, in the next firmware version. "MIDI Thru" will be disabled by default (it is always on in all previous firmware versions) thus preventing from creating accidental MIDI loops.