Several times over the last few years, when I installed an update I lose some work.
Once, I lost ALL the tags on my files 
.
But in the previous update to the last, I lost a Tag Group with dozens of tags in the group.
The tags still appear on the files (thank goodness), but the group is AWOL.
When I right click on a tagged file, it does give the option to Add Tag to Group and it lists that missing Group, but the group doesn’t show on the Tag Library page.
Are you loosing only a particular tag group, or all tag groups or even locations ?
A specific tag group is no longer showing in my tag library.
The tags still exist, and they show up on the tagged files. Interestingly, I can click on a particular tag and still add it to the missing tag group, but the tag group doesn’t show up on the tag library page?
I have had this same issue…it has happened a couple of times now where I open up tag spaces and one entire sub-group in the tag library is missing from the left side bar…the tag still exists as i can bring it up but not sure how i can retrieve the grouping in the side bar…the first time it was just a couple of tags but this last time there were quite a few tags in that group and would prefer not to have to redo them all
Thank you!
I figured out a fix…if you go to the three dots next to tag library and click on it and then export library…all of these tag library sections are shown (even the missing ones)…you can then export the missing section, then import it back in and it comes back.
Brilliant. That worked perfectly. All tags present and accounted for.