What do you mean it doesn't close evenly on the other side?
We've had our 2021 Trailhawk out several time offroad here in the Colorado Rockies -- nothing too, extreme, but everything was definitely flexing. It's definitely made for that. (I guess when you've got a wheel up in the air, that means flexing.
Never had a problem with the liftgate latching on both sides.
That is, no permanent problem. There have been several times when one side or the other didn't latch when we had a lot of camping equipment in the back, which the liftgate would hit when I tried to close it; but some rearranging and/or, um, assertive liftgate-closing fixed it.
On your fishing trips, have you ever had the cargo compartment really full, where the liftgate wouldn't latch by itself? Maybe the liftgate got distorted a bit...
I really doubt it's a body-structure issue...