Hmm.
Unfortunately, while I do make Linux builds since they're easy to do w/ Unity, I don't actually have a Linux machine to test them on. That makes issues with it a little tricky to solve, but so far I've had reasonable luck getting them sorted out.
I have a fair number of Linux players and haven't heard of this before, so it may be something unique to your setup. Let's see if we can figure it out.
Apologies of any of these questions seem "duh" flavored, I'm just going through the list of things that have come up before:
1) Are you using a Qwerty English keyboard, or some other type of keyboard?
2) Does Demon ignore both shift keys?
3) Are there any programs running that might be jumping in front for control of the shift keys?
4) Have the shift keys not worked in any other programs before?
5) Finally, could you tell me what Linux OS and version you're using too?
Thanks for the report, and I'm glad you're enjoying the game.
Hopefully we can get this resolved ASAP, Tower:2-17 are much more exciting than Tower:1.