Right. And that was my first thought, too, except for the mystery of why a GPS would interfere. Does it transmit anything? I thought they were also "just receivers". Of course most receivers do emit at least a small amount of radiation.
The test I was thinking of for this would be to put 2 air punches on the same wrist and try some high-speed passes by a control that way.
I wonder if there's any significant variation between individual punches. I found mine to be quite flawless all 3 days. As long as I put my arm within maybe 6 or 8 inches of the box it always picked up the punch no matter how fast I was moving. On slow approaches it was more obvious how far it reached out when it would start beeping before I'd quite "arrived". I only ever re-punched one control and that was just because my skis were too loud on the icy snow, I forgot to look for the flashing light until too late, and I figured I probably had it but better to be safe. By Sunday I'd seen enough to be more confident about making high-speed passes.
Oh, also, here's the online trail maps
of that place I was telling you about. I've never been there and hadn't actually looked at their map until today, but from this it looks like it has some potential.