3 things could be happening:
1 - you got a terminal board as code page 3 never sends break codes, you will have that is a page 2 adapter (although usually those are ISO, 122% and with a RJ11 plug, although exceptions are common in the world of M)
2 - the M is crashing the adapter, i had that happen to mine, depending on the adapters it will either happen on every keys, every so often or never
3 - either one is dead, although given your symptoms i would not actually bet on that
and barcode readers are just automatic keyboards, i work at IT and i have keyboards recognized by windows as barcode readers and barcode readers and RFID readers recognized as keyboards, no need to read to far into that.
as far as those adapters i had one crashing randomly on my M so they may be a tiny bit random, i had really good luck with tiny dual ports blue ones on ebay and model Ms.
TL;DR: i do agree with Reddit on this one, although if you could get your hands on a cheap/free rubber dome PS/2 kb to make sure it is not the adapter it could also be a good idea