Well it appears that the plot thickens...First of all, let me apologize for causing you to do some work with the file that was attached...I did attempt to load it in Photoshop Mix and I did get the white background...I was definitely puzzled...so I did a few tests and it appears that the problem happens when the iCloud syncs the PNG from my iPhone to the iPad Pro...let me explain the process I took...
Because I couldn't access my Creative Cloud files from Affinity Photo, I used PNG files that had been saved to my Photos app on my iPhone and were synced to my new 10.5 iPad Pro when I got it Tuesday...When I did my test with Photoshop Mix, I used my Creative Cloud version and the PNG behaved as expected...then I used a copy of it that had been synced by iCloud in Affinity and got the white background...after reading the Posts this morning, I went back to Photoshop Mix and used the iCloud version of the PNG and sure enough, it had a white background...confirming what you had verified...so went back to my iPhone and used Photoshop Mix with the copy of the PNG stored in my Photos album...and the PNG loaded with a transparent background...so something happens to the file when it's synced to the iPad Pro...
To confirm I proceeded to repeat the process...open Creative Cloud on my iPhone, opened the PNG file and saved it to my Photos album...loaded the saved PNG file in Mix on my iPhone and got the transparent background...once the file had be automatically synced to my iPad Pro, I opened it in Mix and got a WHITE background...same with Affinity Photo...to see if the problem was with Creative Cloud, I open the PNG file with Creative Cloud on the iPad and saved it in my iPad Pro Photos album...I then proceeded to open it with Mix and Affinity and in both cases the PNG opened with a transparent background...once the file was synced to my iPhone, I opened the file on iPhone with Mix and it opened with a transparent background...
So it appeared that the issue was caused when the file is moved from the iPhone to the iPad Pro by iCloud synchronization...now could it be that when Creative Cloud saves it to the iPhone which causes it to loose the transparency attribute on the synchronization...
More Testing
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I did some more testing to try to prove that the issue was not related to Creative Cloud save function…I then realized that since the iPad Pro was new and had just been configured, the Optimized Storage on the Photo Library was turned on for my iPad which meant that it was not storing original images, so I turned that off thinking that the optimized storage function was compromising the integrity of the PNG…THE ISSUE IS RESOLVED….I also noticed a small quirk when I tried to save the PNG directly from OneDrive to my iPhone…there appears to be some latency between the time the photo is saved to the iPhone and the time the original image is stored to the device…as I was doing my testing, I saved the PNG to the iPhone from the OneDrive iOS app…I immediately proceeded to open it in Mix, only to get the WHITE background…a few minutes later, as I was confirming my observation, I went back to import the file in Mix and now the background was transparent…probably some optimization to make the file available faster, but causes issue with TRANSPARENT background…
So it would appear to be an Apple issue…with optimized storage. I want to thank you all for doing some testing which pointed me to a different path.