Wasn't that what ST-6700 STI's were known for? Seems like there was one particular model around that time period that had more of that issue than others.
But you say this happened with your newer ST-R8000 after just a few thousand. Hmmmmm...
Might it be that the lower limits of your rear DR's are set to where you have to pull it tight against the stop and that's putting some extra stress on the cable in the STI? IMO, DR's shouldn't have to bump against their limits. Limits are just there to keep them from going too far as a last resort when other stuff has gotten out of whack.