I suggest reading my other post first, it’ll only take a few minutes. I’ll wait…
However, if you really don’t want to read it, here’s a quick recap on the initial issue.
In early February 2020, a lot of data was deleted from some clustered columnstore indexes in our PRIZM database. Some of the tables were rebuilt, but 11 tables weren’t since we don’t have maintenance windows, and that would involve downtime. The rebuilds would happen once we upgraded to SQL Server 2019, to take advantage of the ability to rebuild those columnstore indexes online.
Taryn now has the full story and I recommend giving it a read.