Some PNG images are not imported correctly. Is it something in the settings or is it a bug?
Running GIMP 2.10.2 | Windows 10.
Original png with alpha:
GIMP print screen:
The problem is with the PNG, not GIMP. The PNG file is in Indexed colour mode. In Indexed colour mode in GIMP, there are only two levels of transparency - either fully opaque or fully transparent. Instead change it to RGB mode if you want to utilize multiple levels of transparency. Then export.
On opening the original PNG image in GIMP 2.10.2 (using File > Open) here's a screenshot
Click Image > Mode > RGB. Then export as PNG. Here's the result.
As to why this is happening - I suspect the PNG file has been put through one of those online PNG compression websites, which do support multiple levels of transparency in Indexed images. Software like GIMP and Photoshop don't support that in indexed colour mode. I also note that on opening the PNG in Photoshop, it immediately converts the image to RGB colour mode. GIMP doesn't - you need to do it manually.