@bornforphp, thanks for the reassurance, Harshad.
The issue seems to have settled – at least for now, after doing a manual backup on the 1st of May. Since then, the 7 scheduled backups were done for https only – yay!
If the issue re-appears I will wipe and re-configure settings as instructed here.
Thank you so much for your help Bryle and Harshad, I appreciate it!
Alex