It might be popping back as that bolt has rounded or something so isn't holding it where it wants to be adjusted but instead it's defaulting back to the other position.
What I meant is, that it's neither of those reach settings. And the shifter is brand new, so apart from it being fucky from production, I doubt I rounded the bolt
It has 2 reach adjustments, bolt at 1:50 on this video.
https://www.youtube.com/watch?v=GESGpwNLuTQ
It might be popping back as that bolt has rounded or something so isn't holding it where it wants to be adjusted but instead it's defaulting back to the other position.