Created attachment 125652 [details]
Bulgarian flag - png format

SUMMARY
White color becomes transparent when importing image from a file on top of an image with transparent background

STEPS TO REPRODUCE
1. Create an empty image with transparent background.
2. Download an image with white color in it or use the attached one (Bulgarian flag). 
3. Open the empty image with transp. background, then click "Edit" --> "Paste from file..." and select the image with white color in it (or use the attached one).


OBSERVED RESULT
When the new image is imported, you'll see ALL the white areas of the imported image are gone missing and have turned into transparent areas.

EXPECTED RESULT
Importing an image from file on top of an image with transparent background should not turn the white color into transparent one.

SOFTWARE/OS VERSIONS
Windows: 
macOS: 
Linux: Arch 5.5.1 
(available in About System)
KDE Plasma Version: 5.15 until latest (saw this problem for the first time in 5.15 but it was still happening in the latest plasma two weeks ago when I was on KDE for a while)
KDE Frameworks Version: N/A
Qt Version: 5

ADDITIONAL INFORMATION
I made a video showing the problem in details (the link below), hopefully it will be more helpful understanding the bug better than my steps to reproduce. People using full KDE desktop (Linux group on Facebook) confirmed this bug is happening with their kolourpaint as well, so I'm pretty sure the fact I'm using Cinnamon right now has nothing to do with it.

https://drive.google.com/file/d/113vtsTbotOIEFD7XZRs4ACriTAXdQWLr/view?usp=sharing