Pasting image with transparent background does not work properly

[tudza]

Description
I copy an image with a transparent background and paste it into another image. It looks OK, but saving or simply switching to another image tab and back reveals the transparent background renders as white.

To Reproduce
Open two images, one with a transparent background. Paste transparent background image into other image and save. Open saved image to show problem.

Version
Pinta - 3.1.2
Kubuntu - 24.04