I’ve just come across this issue, and it’s never been a problem before.
When I add an image with transparency (.png) as the album cover, Roon isn’t showing the transparency. It shows fine in Windows file explorer and Windows photos - see screenshots below. Of note, other images with transparency do work in Roon. Is there a particular setting or something that needs to be enabled when saving the image for Roon to show the transparency?
If possible, can you please share a screenshot of your image import preferences? With that, I would also suggest clearing your image cache in your settings as well