I looked at one rejected file and one "snipped" file, and the difference was that the "snipped" file had turned little endian, when the original "problematic" file was big endian.
If I understand the spec correctly, PNG files should be in network byte order: https://www.w3.org/TR/2003/REC-PNG-20031110/#7Integers-and-byte-order
So why does the Windows Snip tool generate PNG files that aren't following the standard, and how does it even get any software at all to read its PNG (invalid) files?