Does it work locally? That is, does the proper note play if you use the thing as a stand alone synth? We need to determine if the problem is MIDI ony, or if the key is messed up for all its uses.
Put it on a patch like piano, something where velocity is easily heard. Now start at one end and strike each key twice. Once softly and once hard. Does the thing react properly by playing the proper note both softly and loud, as well as does it continue to sound if you hold down the key? Each key has dual contacts, and if one is stuck (either off or on) it can mess with operation.
When you strike a note, the unit sends a NOTE ON message out the MIDI. When you release the key, a NOTE OFF message goes out. If a contact sticks, the unir will power up while you cannot hear that note, and once going, that note won;t play again until it sees a NOTE OFF. If that note off never arrives, the key goes dead.
If it is ONLY MIDI that is confused, it COULD be a partitioning problem. I may be recalling the wrong word for it, but most modern era keyboards allow you to assign certain sections of the keyboard to one MIDI channel and other parts to another. It is possible that one note somehow got assigned to a different MIDI channel than you usually use, and it is playing, but top a channel nothing is reading.
Seems to me your sound modules receiving MIDI are set to a particular MIDI channel, but I think, isn't there also an OMNI or something like that where the sound module will react to ANY channel sent? If so, try setting a sound module to that and see if it now sees that key. Or just strike that key repeatedly as you set a sound module for each MIDI channel going down the numbers.
I used to do tons of keyboards, but I haven't dug into MIDI problem in 20 years, so maybe someone can be more helpful.