I ues "vi" mode in my shell to edit command lines - but since I updated to Big Sur (or was it an M1?), sometimes when I'd edit a line the cursor would go back a line.
Took me forever to track it down, but Terminal (at least for me) is not setting "COLUMNS", so it defaults to 80.
In previous releases, the shell would get the set and updated as Terminal windows resized.
Posting this in case someone else has the same puzzling issues.
BTW my fix was to just set COLUMNS to 512 in my profile. But really...