Thanks for the extra context!
I did some testing. I set up Twenty Twenty-two and edited my theme to contain a Featured Image block with a Height and a Scale set to “Cover”. I then looked at the frontend in WordPress 5.9.5, 6.0.3, and 6.1 RC 2. What I saw is that object-fit
is erroneously not output in WP 5.9 and WP 6.0 but does appear in WP 6.1.
In other words I think this is a bug that was introduced in WP 5.9 but fixed in WP 6.1.
This doesn’t quite align up with what you’re saying though which is that this worked for you in WP 5.9. The only thing I can think of is that maybe you had the Gutenberg plugin activated but it is now disabled? When I run WP 6.0 and install the Gutenberg plugin I see that the bug goes away.
In any case, I think the solution for you is to install the Gutenberg plugin, upgrade to WP 6.1 RC 2, or wait for WP 6.1 to be released in a few weeks.