Earliest start date, no mods, no custom settings. I always double check that because I basically only play for achievements. Anyway, I started another run with the same settings after deleting some old version saves and the achievement fired, so either it has something to do with obsolete saves (one of which in particular was marked as a "tampered" file with no achievements allowed) or I'm just dumb and screwed something up in the first run.
Due to the lack of reproducibility and further evidence, I suggest to ignore the report.