Hmm, I have a desk job too, but I get up and stretch a lot, and workout at least twice a week. Does this only happen to you on planes? Do you ever sit for 5 hours anywhere else? Does it happen after like a 3 hour movie, or JUST on planes? I did find this interesting article about soreness and flying: https://www.fitfortravel.nhs.uk/advice/general-travel-health-advice/air-travel
Yeah, if it’s not something a little stretching or rehydration can’t fix it seems iffy. Talk to a doctor for sure.