I agree with you. The transparencies do remain transparent for a longer time, if I save them to files. In photos it appears that they only last for a week at most. It's pretty annoying. I don't know why, but I haven't gotten around to calling AppleCare with the issue.
Originally, I was in this discussion because my iPad completely stopped saving anything as a transparency during a roughly 10 day period. Then I discovered that Files would save the transparency around day 5 (I need that ability, so it was a horrible event. I use transparencies every day). Then, without even an Apple iPadOs upgrade, the issue with Photos simply ended and I could again save to photos as transparencies (available for the limited time that you described, thank heavens).
In short, what you described is exactly what I have always noticed. There is a time limit (which I have not figured out), then the .png file is converted to a solid background.
I guess our question is somewhat the same: Why does iOS/iPadOs seemingly need to convert our graphic file types.