Could be a platform issue, but since I really can't see through the mechanism of the mapper code, I can't decide that.
From my view point I could be a subtle dosbox bug just as well.
Moreover, I would wonder if the SDL port (nobody reported a different behaviour for it so far) behaves in a different way.
Let me repeat what's wrong. Only the letter can be shifted, the other keys can't.
In my research I noted that regardless of having pressed shift or not the letter keys return with the same SDL keycode, but the others don't. Which to me is perfectly valid for a SDL implementation.
Also remember that OS/2 runs on the same hardware a Windows or Linux.
Any suggestions?
PS: I will hack now hack my way through the Linux version too see how that behaves.