Worth adding a scheduled build on your default branch, if you do not have one.
It runs the same commit on a timer, so external breakage is detected by the schedule rather than by whoever happens to push next. That turns this from "my change broke the build" confusion into a clean signal that something outside changed, with a timestamp.
It also catches the slow version: something that will break your next release but not your current one, discovered a week early rather than during a release.