Confirmed in Main 4.22 @ CL 4586854
in 4.21 Alpha looks to be about 1 pixel larger than other channels in certain parts of the texture
"The issue is not improved by changing compression settings, using different color depths, different file types"
Result: Alpha looks to bleed beyond opaque portions by ~1px
There's no existing public thread on this issue, so head over toAnswerHub just mention UE-66828 in the post.