What are you doing with your live monitoring that you need the Reaper mixing board and production for? To say that the other way; what's the reason the X32 built in mixer is not acceptable to use for live monitoring?
Not to downplay using the X32 as just an audio interface and running live sound with Reaper running at low latency. But the X32 is designed to fully run live sound and all.
Yeah, downgrading from OSX to Windows will do just that.
You were using the headphone output on the computer before? That must mean you were making an aggregate device of the X32 and your Macbook's build in audio output to accomplish that. Maybe the built in output on that particular machine with the Windows OS doesn't play nice? Was the Macbook headphone output an additional channel because you ran out of outputs on the X32? This is probably where the problem is centered around though. Aggregating the X32 with your built in output on the computer. ie Running two audio interfaces together with sample rate clock sync over the data connection (USB). (The worst choice for sample rate clock sync.)
Do you have spare line level outputs on the X32? You could pick up a headphone amp. That would let you just use the X32 by itself and avoid the more complex aggregate device setup.