I found the trigger, at least for my case: SP activation.
-
Before activating the SP, my Silver Rathalos helm looks fine.
-
The issue starts when activating the SP:
-
And remains like this for the rest of the hunt and in future hunts until you restart the app:
Basically, the helmet “sinks down” during the SP activation animation.
Addendum: I have not tested other weapons and helmets yet.
Addendum 2: It appears that you do not need to use SP while using an LBG. You just need to have used SP before in the current game session, then enter a hunt while using an LBG, and the helmet problem would appear immediately at the start of the hunt. In the Training Ground, this issue doesn’t appear immediately at the beginning, but you can make it appear by dodging forward while using an LBG. Seems to be related to a particular kind of action—it doesn’t seem to occur for HBG, Bow, Lance, Charge Blade, Hunting Horn, Dual Blades, Hammer, and Gunlance. I didn’t test the remaining weapon types.