@olivierlambert @florent Looks like 1471ab0c7c79fa6dca9a1598e7be2a141753ba91 (in current master) has fixed the issue.
But, during testing I found a new related issue (no error messages). Running current XO master b16d5...
During the normal CR delta backup job, two VMs shows the warning message: Backup fell back to a full but it did a delta (not full) backup.
I caused this by running a different CR backup job (for testing) that used the two same VMs but to a different SR. The new test job did a full backup and delta backup correctly. Since there is only one CBT snapshot the normal backup job did recognize that the snapshot was not for the original normal backup job and should have done a full (and started a new CBT snapshot), but it only did a short delta.
As the VMs were off, I guess it could correctly use the other CBT snapshot as no blocks had changed... Just odd that CR backup said it was going to do a full and then did a delta.