I've had a couple FX3s loose zero, but the ones that had the M1 tracked fine for me.

As far as losing zero, the issue may have been the rings for all I know. I still have one of the scopes, and it hasn't lost zero in a while but I also don't shoot the rifle it is on much anymore.

Overall the FXIII was (is) the scope to get if you're going leupold IMO.