I would normally do this myself, but am now disabled and wife has a fit when she sees me on a ladder.
This is the overhang of the roof directly next to the chimney.
One picture is from the underside and shows that the rot is pretty much in the one spot only.
The other picture is from above and I show the roof and cricket of the chimney.
I have not investigated this too much. but can not see where the water is getting in. I also never looked if this was existing before I had the roof reroofed about 6 years ago.
I do not know if I should just monitor it to see if it does get wet when the rains come (if ever).
or if there is a roofer that wants a side job email me at firstname.lastname@example.org