I noticed that plugin WCMp Vendor Staff is running the flush_rewrite_rules() on all pages requests, both frontend and admin. It is a bad practice, according to wp docs: “Flushing the rewrite rules is an expensive operation, there are tutorials and examples that suggest executing it on the ‘init’ hook. This is bad practice.”
I think you already have a draft to manage this, since in the wcmp-sub_vendor/classes/class-wcmp-sub-vendor.php in line 329 has a commented line:
Actually I uncommented the line and the the problem resolved. Can you update the plugin code? Is there any reason to leave that line commented?
If the plugin have a public repository, I can send a pull request for that. Thanks!