PIC 24F output port not clearable
I'd recommend that you check the datasheet and see if there are any errata associated with that particular micro. 

From what I know (in the past), the case used to be where I would have to read/modify/write and then (in some cases) write twice to a port to ensure that it took. 

Aside from that, can you isolate the port to ensure that nothing else is influencing that port pin? If you have a development board with NOTHING connected to that port, it might be worth a try to run the code there and see whether or not it is externally influenced or not.