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this is most likely a bug introduced here

there is already a fix for Qt from nov. 16, and another pr here

you'll have to update your opencv codebase, and rebuild, but for today, try to convert the image back to CV_8U, before using imshow() like:

normalize(dist, dist, 0, 255.0, NORM_MINMAX, CV_8U);
imshow("Distance Transform Image", dist);