Sharepoint modern site icon loses transparency

Hello. 

I have a following problem. I try to change the site icon for team site. It's a transparent PNG...

Here's what happens. At first the icon uploads and displays correctly and the transparency is understood

But after a few refreshes the site icon starts to look ugly, because transparency info is now lost so I get this ugly dark-blue square around... And if I look into the settings of the site, it is broken there too...

Then I replace the icon with the same file and again it works for some time... then the story repeats...

(see pictures)

Icon (still not broken by SP)

How the broken icon is see in settings

Icon (already SP messed it up)

Site logo replaced again with the same file.