I'm not setting the fit mode anywhere, but I tried setting it to None explicitly, and there was no change. Oddly enough, using MultiLine was kind of better, because no labels were clipped mid-label. However, I'm pretty sure the lower of the multi-line labels was intermittently clipped entirely due to the same issue.
I may use multi-line mode for now, because it doesn't look as broken, but some labels are missing entirely intermittently. For what it's worth, I'm using the Xamarin Forms product, not Xamarin.iOS directly.