It looks like this is known issue that has happened to several others before:
It seems that this problem is difficult to reliably reproduce, and hence the dev team has a tough time tracking down the root cause.
If it happens again, gather more information and see if you are able to figure out what reliably reproduces the problem. Explain that this is known bug to your opponent and ask if they are willing to undo and redo the last move several times to see if you can reliably reproduce it.
Does this happen with one-click moves? Double-click moves? Or submit button moves?