I don't know how you feel about it, but all of this seem pretty inconsistent to me. I assume we all at least format /system & clear cache before applying a new release, so how come we have so many different (opposite) behaviours? (needless to mention the random endless reboots). We all run the same ROM on the same device. It doesn't make sense.
The only conclusion I can come to so far is that it's all related to the only stuff that is different from device to device, and that is being carried over between upgrades: stuff stored in /data.
My next test will be to wipe everything (/system, /cache, /data) and flash CM10.1/gapps on a pristine device.
I already did that and I still have the proximity sensor not working. On a pristine device. I didn't test other sensors, just the proximity one.