What should happen at the eaves
Any water that gets beneath the tiles should run down the underlay and discharge safely into the gutter area. Older bituminous felt often becomes brittle where it is exposed near the eaves.
The hidden gutter effect
A sagging strip of felt can hold water behind the fascia. It may behave like a little gutter of its own, except it has nowhere useful to discharge. The back of the fascia then stays wet and rot starts from the side you cannot see.
Signs from ground level
Peeling paint, dark staining behind the gutter and localised fascia decay are clues. They do not prove the felt is at fault, because gutter leaks can look similar, but they justify checking the eaves detail rather than replacing fascia boards blindly.
Repair the water route first
New fascia fixed over a continuing leak will deteriorate again. The roof edge, underlay and gutter alignment need sorting before the timber or uPVC finish is treated as the main job.