My chapters showed "Peak −2.x dB — outside the retail spec" and re-rendering never helped. (fixed in v1.6.3)

That was a defect in our loudness mastering: the limiter controlled peaks in a way that let certain chapters (typically ones with strong "s" sounds) exceed the retail true-peak limit by a fraction of a decibel — identically on every re-render, which is why retrying couldn't fix it.

Update to v1.6.3 and re-render: peaks now land exactly on the ceiling and the check passes.