Willing to bet it's due the the 510 on the mod. Not willing to tear mine apart, but I think it just floats on an o-ring.
If I go to an atty with a shorter pin, I HAVE to bump it back up from below. Then it's perfectly fine. It pushes down from the top if you try to, but it doesn't spring back well. I only notice the issue when switching toppers, and if I move to one with a shorter 510, the bump from underneath solves any issues I've had.
Honestly, that and the fact that it doesn't take a 21700 are my only complaints with it. I don't switch toppers that often though, so the 510 doesn't really bother me. The o-rings to seal from leaking may solve a leaking issue, but the pin doesn't seem to rebound easily enough.