Strange backup behavior

I have my iPhone 11 plugged into my iMac via USB and open iTunes (still on Mojave) in order to do a backup. I intend to do both a local, encrypted backup AND a backup to iCloud. I choose the iCloud option first and then Backup Now button. The backup appears to be happening and takes just a few minutes. When finished, the "Latest backup" info only shows an iCloud backup from about 6 months ago, and the "To this computer" backup shows a couple of minutes prior - when I specifically chose iCloud. Then I chose backup to This Computer, it appears to happen, and the time updates as expected, to This Computer. Then I try again, carefully choosing to backup to iCloud. And again, Latest Backup shows a 6 month old date for iCloud but 'to This computer' shows the just completed backup. What gives? Why can't I backup to iCloud? Before I started all this I specifically signed up to pay for 50 gig per month, tho I was only using 2 of the 5 free gigs. If I don't have enough room to backup to iCloud, shouldn't iTunes give me some sort of warning message??