WP 5.5 caused issues with many plugins that cause the plugins to have JS errors. JS errors in the admin cause unknown effects, including preventing ACF from functioning correctly. WP 5.5.1 has reduced this, however, it may not fix all plugins. You need to find out what other plugins are having issues (deactivate all plugins and reactivate them on at a time to figure out if there is a conflict and it causes an issue), what those issues are (what JS errors are happening) and report these issues to the developers of those plugins.
