I've taken a while to reply myself, so no worries there! Interesting to see at least we have the same issue. What you describe is exactly what has happened to my character, except it is affecting primary and secondary weapon fire in addition to the B button (which should be melee attack). I don't think it happened directly…
Thanks to all of you who are trying to help! Yes, I had already tried deleting that save file, but it didn't fix the issue. I've also went the uninstall / clear / reinstall route. There is someone else having the same problem but with only their B button (see the newer post from yesterday titled "Control Problems").
Hey, someone else with this problem! See my post "FIRING BUTTONS ACTIVATE DEVICES OR ABILITIES INSTEAD OF FIRING". My secondary fire (right bumper) and B button are also functioning as you're describing. My issues cropped up after connecting a keyboard up and idk how it really happened. Did you happen to do the same?
Any chance someone could let me know an update on this, please? I'm still having this issue and support has closed my case on it, leaving it in the hands of the "devs".