Some users status changed from “Verified” to blank

One user gave me a heads up on an email with the following content:
“Hi {{username}},
Your registration on {{blog_info}} has been changed to pending.
Sorry for the inconvenience.
You will be notified after it is approved.
Thank You!”
In an attempt to troubleshoot, I looked at the contents of the “usermeta” table and found a possible common denominator for affected users. For the affected users, the key “ur_user_status” had been added with the value NULL.
By simply deleting these few records (with the key “ur_user_status”) from the “usermeta” table, the users concerned have regained the status “Verified.”
Can someone tell if the key “ur_user_status” is added by User Registration, under which conditions this key is added, and what the purpose this key is?
In my case, only a few users have been assigned this key in the table “usermeta.”