Using 5.3; I just came to this solution in reverse.
I needed B8G8R8A8 and could not get it; changing compression settings appeared to work, but did not enable the appropriate functions that would indicate it was working.
3 hours of tweaking later, turns out if your png resolution is in power of 2, UE treats it as whatever DXT1 is no matter what.
I just had to change the resolution of the png to be odd numbers.