Is it a mistake that Set Anchors for /commaturnedabovecomb puts _top on the cap line (instead of on the midline like all the other [lowercase] diacritics)?
Yes, that is a mistake. Will file a bug report. For now, please manually drag the _top to the x-height.
I believe that’s based on the case setting. commaturnedabovecomb does not have a case set, so it gets the body height or ascender, in this case. We’ll review the marks definitions for that.
True, that’s what it was. Fixed.