The Application looks to be heavily skinned and probably re-implements basic Windows controls itself, making it IMO rather difficult to diagnose except by whomever wrote it.
The selected text highlighting gray for example isn't a Windows thing so it's hard to know what it means. It could mean the textbox doesn't have the focus but has a selection, for example. it could mean that $20 is higher than some other setting or maximum that is part of the game.
it seems like perhaps they have some special functionality based on Num Lock as well.
If you can select text correctly in other applications, then it is 100% on this app. Also, they lied when they said nobody had reported it, since you did!