Emax

You can do a Midi out test (using the MIDI SCOPE or any external midi device) just to confirm that there is not sent a NOTE OFF command

Assuming the spring is doing his job, that is almost sure a faulty trace on the keyboard PCB. You will need to remove the keyboard printed circuit board and check the traces involved on that key. Sure the Key Off trace has some problem (Maybe a cold solder or so).

It is a common issue. I had two emaxes with that same problem. In one of them I had to re build three traces showing corrosion. In another, a whole octave started to fail. Re building traces fixed it all.