Hello. In the current 6.x-1.0 version of the module, within the function requestinvitation_send_invitation(), the following line of code exists:

variable_set('invite_subject_editable', TRUE);

This changes the behavior as specified within the invite module. On my site, I never want users to have an editable subject so I turn that checkbox off in the invite module. This module then turns it back on with that variable_set.

I'd suggest the variable_set be removed. I can make a patch if you like however it's just deleting that line.