Clean out line and soft ground
The broken cap would not generally result in liquid spilling to the ground and saturating/softening the surrounding earth unless there had been a stoppage/blockage and the entire line backed up.

If you have not had a blockage, consider whether the soft ground could just be the result of the recent excavation required for the install. After the line was installed in the trench, the builder would normally back-fill it with the same dirt that was removed but perhaps did not compact it enough.

If you need to reassure yourself that the line has not been compromised, the easiest and quickest solution would be to just hire a plumber with a drain inspection camera.