CorytonSLF
I am just humble SLF, but quite a frequent flyer, and I have never seen Cabin Crew leave a seat in a none upright position for take off or landing. Perhaps the passenger had a medical issue which precluded the seat being raised as that is the only thing that springs to mind for them not doing so, or maybe the seat had a tech issue that had developed during the flight. I am sure someone who actually is CC will be along soon to give you a better answer to your observation.
Cheers
Mr Mac