Lose Arch in Back when squatting

[tdhill87]

When I do bodyweight squats I am able to go ATG but once I pass parallel I lose the arch in my lower back and it rounds pretty substantially when I'm sitting at the bottom. It isn't as pronounced when I squat with the bar.

Any specific reason why this happens?

[Brodie Butland]

If you're maintaining acceptable arch when squatting with the bar, what's the issue?