Whopper patties IMHO are the worst fast food burger patties. Sometimes they're over-microwaved and on the border of inedible. I can't imagine an impossible burger tasting worse.
They do get flame grilled initially, after being pulled from the freezer, but then they sit in a heated drawer for an amount of time. The only reheating they get is a quick microwaving for maybe 5-10 seconds, and then they're off on their way. (This is all unless you ask for it fresh, in which case you'll wait for 2-3 minutes while you get a fresh patty.)
In addition, the people working in the kitchen are supposed to toss any extra patties out after x hours, provided they don't just press the ignore button when the timer goes off and do nothing, which is what I saw 99% of the time when I did this job like, 15 years ago.
So if you come at an odd hour, you may be getting a particularly old patty. Granted, things may have changed in a decade and a half and I worked in a very low profit store in a rural area, so my experience may be much different.