I’m having the same issue! At first I though it was my fault (wrong color space or something).
In time, though, I’ve come to realize that the photo is kept with a transparent background for some time when I airdrop it from my Mac.
I believe iOS’ storage optimization has something to do with it, since it stores the original file on iCloud and keeps a light low res version of most pictures. Maybe somewhere along this process the “transparent chanenel” is flattened.
Ive come up that theory because whenever I retransferred the “png” file to the iPhone, it wouldn’t work until I deleted and removed from the “Deleted” folder. Then, after syncing the deletion with iCloud I would transfer again and the transparent background would working… temporarily 💩 haha..
I know the process of choosing which images are kept offline is automated. But iOS should give us the option to pick and choose a photo to be kept on the phone.