Hello Larry Cohen,
The short answer: yes, it is "normal" that Time Machine doesn't look back before Big Sur was installed.
The reason why you cannot see your Catalina backups from your Time Machine drive, is because Apple changed the file format for Time Machine backups to APFS (Apple File System), so it is the same filing system used in Catalina and Mojave, and continues with Big Sur.
At any rate, prior to the update to Big Sur, Time Machine backups could only be kept on external drives formatted as HFS+ Extended Journaled (MacOS Extended Journaled). Big Sur is not capable of reading those files, since it and the Time Machine backups it creates are APFS.
What I had done prior to updating to Big Sur, was to make a copy of my very last Catalina backup, just in case my update to Big Sur did not go so well, and I would be able to go back to Catalina if need be.
So far, I have not had an issue with the update. So, after finding that everything was still working properly, I erased my Time Machine drive and reformatted it to APFS, just so I could have more room for the Big Sur backups, plus I no longer needed the older Catalina backups, since Big Sur had made them obsolete.
Here is an article which you might like to see: Back up your files with Time Machine on Mac - Apple Support
I hope this helps.
Good luck to you!