This seems to be very similar to this thread. In the last post I wrote:
I came across this thread on GitHub https://github.com/osmc/osmc/issues/434 where it was suggested that Kodi was (still is?) capturing the events and doing nothing with them. What’s the current state of play @sam_nazarko?
Please stop Kodi from the command line (systemctl stop mediacenter) and see if the button then works on OSMC.