Your workaround seems to work for DLSS 4 in Unreal. I implemented it here at the cost of X2 GPU cost !
With previous version of the unreal plugin, alpha upscaling was enabled since 3.7. Actually it was well announced in the changelog. And I never had a problem with it was replaced with this new DLSS 4 plugin.
So I still wonder why it didn’t work for you in 3.5 or for Roel in 2024 -_-
Yet more puzling, accoding to the DLSS 4 UE Plugin changelog, alpha upscaling was enabled since September 2025!!