Hello,
The Handle XML mime type option under Page Cache settings seem to be not working as expected in the latest version of the plugin (version 2.2.1).
I did try on a fresh WP (5.9) install with default theme and no plugins activated. The posted data on the Page Cache settings page sends the pgcache__cache__nginx_handle_xml as an array with 0 and 1 (probably boolean) value sets. Should have been string or bool though.
Thoughts?