This PSD file has clipping masks, which are not available in Paint.NET.
In general, the PSD plugin only implements PSD features that also exist in Paint.NET. However, masks have long been an exception to the rule. Masks are a heavily-used feature in PSD files, and discarding the masks would cause a lot of files to look wrong.
Clipping masks are similar to layer masks, and they're even in the same menu group as the layer masks in the Photoshop UI. This argues strongly for adding clipping masks to the PSD plugin.