Contacts suddenly removed from Groups

[shanghaiknight]

I use groups extensively to classify contacts - I have 2600+ contacts.

I recently went into look at a group, and that group (which previously had over 1500 contacts, is completely empty. All contacts were dumped into "No Group" Multiple other groups are completely empty as well.

Last few days I've done some things, like use Samsung phone "Merge Contacts" but I don't think that was the issue. I used my tablet and merged contacts in Fastmail, and I think that was the triggering event.