I assume this is something new in 6.7, because I just noticed it yesterday. (I also just got a new Mac this week and have only observed this issue since I switched, but I’m running the same OS version as I was on my old one.)
I’m finding that if I upload a PNG with a transparent background (which is the only reason I use PNG most of the time) in the Block Editor, it’s getting converted to a JPEG with the transparency either turned black or some kind of distorted color blocks from the foreground elements of the image.
If I go into the Media Library and look at the images that were uploaded, the Original image link is not to the original PNG version of the image, but a HEIC file with a name like tempImage1fPWE9.heic and then that has been converted to a JPEG (e.g. tempImage1fPWE9.jpg) for display on the site.
If I upload the PNG directly in the Media Library, it uploads normally. This problem is only happening in the Block Editor.
I saw in the release notes that 6.7 has added support for HEIC images, but I am not uploading HEIC images, they are definitely PNGs on my computer.