Ah, you mean that if you delete/remove a button it can’t be added.
I just tested with my ‘Forward’ button, changed the binding no problem. You can also set the binding to ‘none’ if you don’t want any action.
However, if you remove that - then it can’t be added again…
So I’m wondering if I can figure out which specific file holds this setting, otherwise it might simply be faster to restore a snapshot.
So yes, your temporary (noob) solution would be to restore a snapshot or backup and then not remove them again until this bug gets sorted.
Meanwhile, report the bug 
I’m looking further now - you can get a list of your most recently changed files (if you don’t know which file) by doing this:
find ~ -type f -mmin -10 -print0 | xargs -0 stat --format='%Y %y %n' | sort -nr | cut -d' ' -f2-
So looks like there’s a ‘kcminputrc’ file, looks likely…
Rename that to ‘kcminputrc.current’ and copy another back from a snapshot (about 12 minutes old) and then put them in Meld… Looking in Dolphin, I can see that’s what changed (and the size was different too).
So confirmed, that worked and I have my two buttons back… but then I need to re-do the bindings ('cos the old bindings came back too which I messed with whilst reading your post).
So hopefully you take snapshots and/or backups.
Oh, and did I mention - you should take regular backups and snapshots…
