incremental Time Machine backup going on for days and then no new backup file is produced
I am backing up a 1TB SSD on my iMac and a 3 TB external HDD to my 10TB external HDD which is configured as my Time Machine drive.
Usually I see a backup ion progress showing "backing up 4GB of 20GB" or something like that when I click on the Time Machine icon on the menu bar.
The current backup started yesterday and is still going. Now is just says "Backing up 2.08TB" but it doesn't say how much has yet to be backed up.
This happened once before when my Time Machine drive got corrupted and I had to reformat it. That isn't the case now. My Time Machine drive in Finder looks pretty ordinary.
It feels like Time Machine is doing a whole new complete backup. Then it suddenly stopped and now says my last backup was two days ago. No new file was created on the Time Machine drive.
What would cause it to look like it's doing a huge backup, especially when it's been doing incremental backups every day?
Screenshots attached.
iMac 2014 Big Sur MacOS 11.6.5
iMac Line (2012 and Later)