Hi,
I had the exact same problem as yourself except I am running Kodi on an Odroid C2 unit with a Harmony remote.
I fiddled around with the keymap add-on (that's configurable depending on you platform) as well as logging keys etc. See Keymap - Official Kodi Wiki for details but in the end, I found that the Kodi 18 upgrade must have moved the remote.xml in my .kodi/userdata/keymaps to a backup version (remote.xml.bak.0) effectively removing the customization placed there in there from my orignal setup.
So, all I did was simply copy the file back to remote.xml and my context menu button was restored on my Harmony remote.
cheers!