The shading on the full data panel which indicates the axis range on the zoom panel doesn't fully cover the axis range actually used in the zoom panel.
df <- data.frame(weight = c(1.2, 2.5, 3.6, 4.0, 4.5), outcome = c(20, 2100, 200, 10, 2500))
ggplot(df, aes(weight, outcome)) +
geom_point() +
facet_zoom(y = outcome < 300)
The shading on the full data panel which indicates the axis range on the zoom panel doesn't fully cover the axis range actually used in the zoom panel.