Huh... it looks like the built-in PNG converter straight up crashes on
some, not all camera roll photos, judging on some tests I did.... go figure. Hopefully they fix that bug. I switched it with JPEG (highest quality) and it works. Try it here:
https://www.icloud.com/shortcuts/04a633628cce450da9bcb61512e1b080
You could also switch with TIFF for highest quality, but it will be large files, up to you.
I don't see why that shouldn't work, just pick a file type that supports transparency (so don't use my JPEG option, pick PNG or TIFF and hope their converter doesn't crash...)