I just found out a problem with TM if you have upgraded from Mojave or earlier and had external drives (I have a few SSD). When I upgraded, it converted my main HDD to APFS, and ONE of the SSD partitions, but did not touch the other 3 SSD partions. As a result TM never backed those up once I upgraded to Catalina.
I just converted the drives to APFS, and hope TM now handles them correctly 
TM normally only backups to hfs+ drives…
I don’t think apple will ever update this, that is a reason may be why they stopped the time capsules ?
Oh truly that was the reason… what confused me is why ONE of my SSD changed to APFS during the upgrade, and the others did not… but it is all good now, All are APFS, and all are in the backup cycle again.
They already changed it 
Am not talking about the drive the TM is saving TO, I am talking about the drives it backs up FROM.
yep for bigsur only.