Further to my last message, we've switched to using https://tranco-list.eu
instead of the Majestic Million and added some additional pre-flight checks to the updater that collects and builds the domain whitelist from this data to prevent this issue from reoccurring.
The new whitelist data was then used to remove any domains that should not have been present in the domain blacklist, which was then manually checked and the zone is once again being built, so everything is now back to normal.
Once again, my sincere apologies for any inconvenience this caused.
Senior Product Owner