Could not find documentation on red-shaded area around smooth spline

[MikeACG]

Hey there! I was wondering what exactly is the red-shaded area that I see in the following plot and why it is so spiky looking:

The context is a big Poisson GLM and this is a predictor vs residuals plot for 1 of about 40 predictors. By looking at the code of the plot residuals function I figured out that for large datasets DHARMa switches from quantile regression to a smooth spline to draw the dashed red line. However, I'm unable to figure out what the area is. I see some polygon being drawn in the function but seemingly only when quantile regression is used so I'm a bit confused.

EDIT: I forgot to say that I'm running version 0.4.6 on R 4.3.0. Also, for other (even bigger) datasets the red-shaded area seems to not be drawn at all. First I thought it was just because the area was so small I couldn't see it in the other dataset but it really does appear like its not drawn at all.