Originally Posted by
reconnaissance
It’s often caused by a combination of cable relaxation (stretching) with use and frame flexure as you power down. Frame flexure makes minute changes to the chainline. Minor tweaking of the cable adjustment barrel is your first thing to do, then test. Only make one adjustment at a time, to rule out other effects.
That's a first week thing, not something sudden 3000 miles later which sounds like a breaking cable.
Cables don't stretch. The housings sometimes settle a little.