Not well acquainted with roofing. I think the ceiling is kind of part of the roof here on this old home with beams and not the contemporary drywall ceilings, see photo.
If you look closely at the photo, the middle column section of the ceiling boards supported by the beams seem to be sagging down a bit, though not a whole lot. The columns to the right and left are more normal but might be a bit saggy at some parts.
So my question is whether that sagging is normal and ok expectation by design or aging of home, or if that is a possible indication of roof issues? Shingle roof in that area seems fine above, but can't tell for sure unless you tear up the area to see conditions underneath.
it appears that you have undersized rafters which means then tend to sag around the middle of their length. at least that's what it appears from the photo.
take a level and press one end firmly against the bottom of the rafter near the middle of the rafter. if there is no sag in the joist the other end of the level should be against the joist. if it's not, then the joist is sagging.
Nothing to lose sleep over unless you plan on having a large gathering on top of the roof.
"You get what you pay for, and sometimes free costs more!"