There were mention(s) in this Support article about Big Sur and
suggestions about Catalina; as the definite trend started there.
• Back up your files with Time Machine on Mac - Apple Support
https://support.apple.com/en-gb/guide/mac-help/mh35860/mac
These kinds of pages can differ in content by chosing another macOS
in the version selector; for persons with pre-Mojave or Catalina the
changes for some topics can be revealing from on portal such as this.
This, within Big Sur section; above:
Important: If you create a backup on a Mac running Big Sur, the back-
up isn’t compatible with Macs running older versions of macOS.
Companion article in Catalina version, as linked through Big Sur:
Note: If you create a backup on a Mac or partition with macOS
Catalina, you can only use that backup on Mac computers or
partitions with macOS Catalina.
This covers Catalina while issue/change persists in later Big Sur:
• Time Machine and backing up in Catalina - The Eclectic Light Co.
https://eclecticlight.co/2019/11/11/time-machine-and-backing-up-in-catalina/
Viewed few others that spelled out changes or differences (while
I'm not necessarily needing those now, especially for time being.)
The Eclectric Light article has pointedly shown links to Apple Support.
Here's another found there (seems 'en-GB' sourced from userguide)..
• Change Time Machine Configuration preferences on Mac - Apple Support
https://support.apple.com/en-gb/guide/mac-help/cpmh0053/mac
note:
If you upgraded to macOS Big Sur on a Mac that uses a Time Capsule or
other network storage device as the backup destination, your existing
backups are also upgraded and can be used only on macOS Big Sur. New
backups that are created can be used only on macOS Big Sur.
So that may pre-explain some issues that some users may experience.
Kinda like a low-hurdle ~ after high speed stretch, of expectation; and
one of those huge water puddles; that could trip a jumping horse?!
Good luck & happy trails!🌞🎣⛵️🌤🌜