Kenmore: freezer working, refrigerator not due to frozen air return...replacement needed?
In my experience, "frost free" refrigerators frequently benefit from an occasional through (manual with carefully applied gentle heat, or "move all the food somewhere else and leave open for 24-48 hours") defrosting. I've honestly had far less problems with problematic frost build up in "manual defrost" refrigerators, (which normally have separate cooling coils for the freezer and fridge rather than air ducts that bring moist fridge air into the freezer to make frost) but you can hardly find one of those new these days.

Anyway, I'd suspect that you are correct, and the door got left open a bit. Or leaks a bit.

I don't know what you mean by "checking the door seals with your hand" - the "standard" test I know is to use paper (typically paper money) closed in the door and make sure there is resistance to pulling it out all the way around.