Yeah, I see that. It happened when I made the post though - the files are not corrupt.
I *think* I may have figured this out. My Moderator group has the permission "Can Moderate All Forums (is Super Moderator)" set to yes - so in effect they are equal to SuperModerators (id:5) and so by design can't be edited by them.
Sorry to take up your time with this oversight.

Thanks for all of your help!