The core of a jet engine operates at thousands of degrees F...takes awhile to “go out”/cool down....(after all fuel is cut off). ;-) Yep, not want you want to see esp. in a 2 engine aircraft...fortunately the 757/767/777 series are certified by the FAA as “ETOPS” (safely fly four hours or more on one engine). See:
https://simpleflying.com/etops-certification-requirements/
I realize I was probably wrong to assume that the jet engine was running just because I could see the turbine blades rotating. If the engine was running, I probably would not be able to see the blades at all.