PNG does not support “multipage” images.
MNG is a PNG variant that supports multiple images – mostly for animations, but it’s not a real PNG image (diffent signature/header), and has never become popular.
APNG is a similar attempt, but more focused on animations – it’s more popular and alive, though it’s less official – it’s also PNG compatible (a standard PNG viewer, unaware of APNG, will display it as a single PNG image).
Another possible explanation is that your image is actually a TIFF image with a wrong .png extension, and the viewer ignores it.
The only way to know for sure is to look inside the image file itself (at least to the first bytes)
Update: given the pngcheck output, it seems to be a APNG file.