The selection for keys seems to be not working correct.

I'm using:
Forge 31.0.1
Minecraft 1.15.2
Controlling 6.1.0

Screenshot

Issue: I select the [X] for keys and enter only "a" (or other letters) but controlling shows me also other keybinds instead only the ones with "a". with digits it works fine but not with letters.

That is because it is using the name of the "key", so if you select "key" and then type in "left", it will show all keybinds who's key name has the word "left" in it, like this: