Bizarrely, the contrib theme I am using had .css on all of their other css files in the yml, but no .css on one of them. That was causing an error.
Also, our Drupal 9 sites with the same contrib theme aren't throwing an error, but the sites we have updated to Drupal 10 are.
In any case, I found your comment very helpful, @Freddy Rodriguez and wanted you to know.
Tested and working! Thanks for this