Well, I usually start debugging by doing a Cmd-R.
Has it ever worked in 2018r3 or is it intermittent? Did you see if the entry was in the shortcut editor still?
Intermittent. What else?
I don’t know, “doesn’t work any more” says to me it doesn’t work any more, not that it works sometimes, you just said it doesn’t work any more, so I was trying to gather some more info for clarity 
Like I’ve asked twice now if the entry is in the menu editor and you haven’t answered 
I can’t really help with that one as I’ve not come across it, there’s some mac tools out there for screen recording as you work, when something happens you can trigger a recording of the last x minutes, I’m sure someone can post some links. Getting a video of it happening sure would help to narrow that one down.
It might be best to make another thread about it or a feedback ticket with some additional info, like when it happens does the Run button work or the menu option for run or does it vanished from the menu editor and see if others are experiencing a similar issue.
Happened again, I’m starting to see a pattern here, I knew I wasn’t mad posting in here all the time 
Happened again. I’m wondering if this is something to do with coming back to the PC after the screens have powered down or its been in sleep/hibernation, clutching at straws there. If I remember I’ll open all editions before I go to sleep tonight and see which if any break as I had a 2016r3 open that didn’t but the 2018r3 did. Hmmm