I did some tests with white and pink noise a few days ago, after noticing the same issue. I’m still running into this problem on 4.19.2 with audiomixer activated. There’s a hearable roll-off on the higher end of the spectrum, even at the highest quality settings. No roll-off when compressing to ogg Vorbis or ogg Opus with Reaper for example.
FWIW, I tried playing back the same sample in Wwise and got a similar result to Unreal when using ogg at max quality.