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The crash bar is completely firm, so even if water froze in there, it would rather push the skin outwards, and not the bar against the skin.
Given the regular temperature changes mentioned by the OP, and assuming that the material of the skin and the bar are different, i.e. have different expansion coeficcients (as well as the crash bar being part of a rigid structure), I can imagine that the door skin pulls itself against the crash bar during cold weather - this would also explain why the crease is getting worse.
As to why it is only present on one side: Subaru's factory tolerances are quite large, and actual values even differ from side to side on some parts.
Just a theory, but makes sense to me.
You can easily remove the door card and check what is going on inside the doors. I suggest to check both sides and look/measure for differences.
Just watch out for the wires and the mechanisms for the door handle and door lock when Lifting the door card up.
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