We need to make sure the add_rewrite_rule is running each time otherwise the rewrite roles get lost if other plugins flush_rewrite_rules.
Now the um_flush_stop check is moved to the add_rewrite_rule function to make sure add_rewrite_rule is fired.
If the option "Stop rewriting rules on every load" is set to "on" and a plugin flush_rewrite_rules your add_rewrite_rule will get recognised.
I also added a filer 'up_rewrite_flush_rewrite_rules' to allow plugins to stop rewriting without manipulating the option array.
I hope this makes sense for you and help you. My english is not the best let me know if I got something wrong here or you have any question left. I like to help.
Sven Lehnert
03a2ee8fbd