From what I’ve experienced with this unexplainable problem is…
The multiclip can be opened, played and then marked in the viewer and those markers will be retained as long as you never switch your camera to something other than the “master” or “default” camera that’s selected when you first open the multiclip.
If you drag that marked multiclip into a sequence and then reopen it in the viewer – the markers seem to stay regardless of what camera you the switch to thereafter. So it appears as the problem is with the original multiclip being opened from the bin into the viewer.
For me, I use the inital multiclip (that I mark in the viewer) as an outline for my footage. I then just select the pieces I want (by clicking my marks that I see in the bin) and drag that section to my timeline.
It’s kind of a tedious workaround, but gets the job done without adding any real time to the edit. You just have to be conscious of where/when you’re clicking.
Put this into the ever-expanding list of “unexplained FCP errors.”