Monterey Time Machine backups: A serious incompatibility issue with previous OS

What happened:

I backed up my MacBook Pro to my usual outboard usb TM drive before going on trip. (This drive was used before I upgraded from Big Sur to Monterey, so backups using both OS were on the drive). On my return, my wife spilled a cup of tea into the keyboard resulting in logic board damage. While investigating repair/new purchase options for the laptop, I tried to restore data from the backup to an old machine (running Big Sur), but the backups were not visible on the drive! Disc Utility showed there was 900 gb used on the drive but even after I ran disc repair, they were not visible. I tried the drive on another machine, also running Big Sur, same issue. I resorted to recovery software and was able to retrieve the back up (12 hours). Meanwhile, a laptop repair company was able to repair the computer for $350 (Applause). I hooked up the offending Time Machine drive today, planning to format it and start again, BUT..... you guessed it, there were all my backup visible on the drive! Discussions with expert Apple technicians were fruitless during this episode. While I was just inconvenienced, as most of my stuff is online and I backup media without using TM, this is a pretty nasty gotcha. I am still going to format the drive and start again and I will check if the data is visible using Big Sur.