It looks like you’re passing the string ‘EP_PERMALINK’ instead of the constant (which is an alias for an int) EP_PERMALINK. It’s possible that PHP 8.1 is stricter than 7.4 when it comes to variable types. WP_Rewrite::generate_rewrite_rules() definitely expects an int for the 2nd parameter ($ep_mask).
There must be a plugin that is affecting your htaccess file. (It shouldn’t.) The htaccess WordPress directives are rewritten when you change the permalinks in Settings. (Which is a good way to get them back if they aren’t there.) I’d do a text search of all plugin’s code for ‘htaccess’. Once you find the offending … Read more
Rewrite rule giving 404 error
Rewrite rule to index.php in theme subfolder
Same slug for Custom Taxonomy archive and CPT archive – Rewrite rule not working
I found some documentation here. It looks like this is accessed when saving the permalink settings under Settings->Permalinks. It looks like this works in conjunction with the rules stored in .htaccess.
Rewrite URLs in a Custom Post Type with Multiple Taxonomies
Rewrite rule to simulate page hierarchy results in 404
You can have a different slug for the archive by setting has_archive to a string: register_post_type( ‘whatever’, array( ‘has_archive’ => ‘whatever/results’, ‘rewrite’ => array( ‘slug’ => ‘whatever’, ), // …etc. ) ); I’ve tested that and it seems to do exactly what you needed.
Pretty permalinks and rewrite rules are the same thing, so you need to have them turned on for custom rewrite rules to work.