Thanks, this is sorted now, there's kind of two issues. One is fixed retroactively, one isn't.
The first issue applies to anyone using custom fields in MG 1.1 prior to upgrading to MG 2.0. The category table field_cache format changed and we didn't update it during the 2.0 upgrade. This isn't fixed retrospectively but you can do so by just going into any field and hitting "Save". I suspect most people won't have been affected because at some point if they've added or changed fields then this will have updated all of them to the correct format.
The second issue applied to the xfmg_media_edit template - it wasn't filtering out fields that didn't apply to the media's category, so essentially all fields were showing on edit, even if they weren't allowed in that category or album.
These changes will be in XFMG 2.0.6 and/or XFMG 2.1.0.