Why do you have background rendering on? I'd turn it off, wastes disk space, wastes CPU time, shouldn't be needed in most cases. This is a bug. But again, why are you keeping background rendering turned on?
99% of the time you don't need it. It is only there for situations when your Mac is too old or underpowered and can't give you good enough playback for editing. Turn it off, delete all your render files, be free.
Because Apple is dropping the ball constantly these days. Are you using native only plugins or red giant/neat etc.. as well? 10.4.4 introduced this re-rendering bug and have been going mute on the feedback forms. They used to reply once in a while, now they run on auto-pilot not fixing any bugs that show up. Report it as I have and hope for the best.