Hi,
is there any way to make opaque selection working as in other graphics programs? Below I attach some test examples - simple shape rendered in Blender with transparency on, then saved as png. Then loaded into Krita/Affinity Photo/Paintstorm Studio/PShop and selected transparency. Result in Krita is little disappointing…

Krita:



Affinity Photo:



Paintstorm Studio:

Make a bug report and attach the original png file to it. Comparisons with other applications are not useful, so you don't need to attach those images.

By the look of it, is clear that the original png's transparency is not "clean", as we can see a base where the cube is placed, was that a flat object that was used on Blender to render the shadow?, that tiny amount of opacity of the rest of the plane is likely not 100% transparent and obviously selected on Krita. I'm not sure how you going to use/paint your pieces or if they are going to be too complex, my only advice is that there are other options to select, a flat straight cube like that can easily be selected with the polygonal selection tool, you can also try to play a bit with the "fuzziness" parameter that will increase/decrease the threshold of the selection. Lastly, it will always be much better (and easier) to get the png clean before using it, that will save you from headaches on the long run