iPhone keeps saving png files to jpeg

Hi, I’ve recently purchased a new iPhone and the problem is still persisting which is highly annoying but I use a watermark for my buiness pictures when posting on social media and use a bunch of different apps but it used to save photos fine in the corect format then all of a sudden it decided to change all saved watermarks into jpeg. And seems

to be doing the same on new phone? The original file that is saved into “files” on my iPhone is a png but as soon as I save it to photos it changes it! Hope this makes sense any help will be amazingly appreciated!!