You can try to check to see what is the correlation between these users and the number of users in the review pipeline and see if you can figure it out.
I would do this for you; but we don’t have the same class of users in our review queue and when we migrated we turned this off. Actually, we had it turned on (same issue as you) and then we just reran the migration with it turned off in the settings (as Jeff suggested to you earlier).
Having said that, you can try to correlate by looking at the total number of users you are seeing in the review queue against these queries:
For example, our app container name I am looking at now is named: “socket1”:
ubuntu:# docker exec -it socket1 rails c
 pry(main)> User.where(active:false).count
 pry(main)> ReviewableUser.count
If I were you, I would run these queries above and record the numbers and compare them against what your admin UI shows as the number of users you need to review. If you see a strong correlation, you can example those models to see what you need to change; and then you can try it out on one user (not against the entire DB table).
Then, if you can clear the “reviewable flag” in this way, successfully for one user, and all works out, you can do more.
In addition, as you know make sure you have a full working backup before making changes to the DB with a query.
Finally, you might want to consider setting up a staging / development instance, restore your current DB to that instance; and then you can try without fear of causing damage to your production system.
Hope this helps.
PS: Don’t forget you can also review the code on GitHub and search for keywords there, etc.