The PDF in question uses a clipping technique that until today I was not aware off. It's been in the spec for some time under Marked Content. In very specific cases the marked content is used to define a clipping path. Or as the spec states.

Some PDF path and text objects are defined purely for their effect on the current
clipping path, without themselves actually being painted on the page. This occurs
when a path object is defined using the operator sequence W n or W* n (see
Section 4.4.3, "Clipping Path Operators") or when a text object is painted in text
rendering mode 7 (see Section 5.2.5, "Text Rendering Mode"). Such clipped,
unpainted path or text objects are called clipping objects. When a clipping object
falls within a marked-content sequence, it is not considered part of the sequence
unless the entire sequence consists only of clipping objects. In Example 9.2, for
instance, the marked-content sequence tagged Clip includes the text string
(Clip me), but not the rectangular path that defines the clipping boundary.

We currently do not do anything with marked content tokens. In the PDF in question, there are for images painted, the first is the background image that should be displayed followed by three more images that should be clipped using a clipping path. All for images are the same size and because we don't clip only the last image is shown at render time.