Yes it fades in (the opacity value goes up, but there’s nothing to show yet because there is no layer at that point).
Yes, the expression works fine, I’ve used it thousands of times and I’m using it in the same composition with other similar layers (precomps) without issue.
It’s just this one layer that is misbehaving. The in and out are the beginning and end of the precomp layer, I’m not trimming it in any way, which I’ve done in the past too.
I think it’s a weird glitch, I will report it to Adobe later today.