Just tried completely uninstalling the Steelseries Engine, shutting computer down + pulling out and plugging the keyboard back in and now the game is running fine. Maybe it is due to the Steelseries update the other day? Still I don't know why it would have a delayed effect. Should we contact Steelseries about this now or leave it to you guys, because this seems to have only affected Bannerlord. Even Warband is fine.
Didn't work unfortunately. However there was an update to the Steelseries Engine on the 21st of June, however the notes say it was only adding support for new devices.
As I said above, the keys will work on the first input but not after that. The keys break after that first input and then the only way to fix it is to delete that file, whereupon the keys will break again on the first input.
So I just tried your workaround, twice. It seems that it will work (Ctrl + click) for each input once and then not work again. I.e. you hold ctrl + click to sell a bunch of items, and for as long as you hold ctrl, its fine. But the moment you try a second time, it no longer works. Same goes for shift+click and alt. Restarting game/exiting and rentering menus doesn't help, only the workaround you just posted.
I should have added that I am also on a Steelseries keyboard (Apex).