Outlook Indexing strange behavior

Indexing shows indexing complete for 1,554,885, and when I check the indexing status periodically it's 0 items remaining to be indexed almost of the time.

The issue is sometimes it's showing suddenly around 300,000 items remaining to be indexed! this happens after receiving just a few emails and I am sure it was 0 before 15 minutes... I don't think it's a bug resetting the whole index... Is that normal, or do I have a problem with my indexing?

I have Microsoft 365 Personal: Outlook - Windows Version with the latest update.