Hi,i am tired of writting posts abouth this issue.
Hi-Res is broken with this update and It seems to happen on all Samsung devices with Pie.
I tried everything on my phone (S9 with exynos) ,its full stock not even rooted.
i don't know why you keep saying that Hi-res works on the s9 Pie update,there are several people with the same issue who have post it and for a few weeks (several times myself).
You are on a Pie Beta, you should try upgrading to the official Pie rom thats when Poweramp doesnt detect Samsung UHQA
Also,i never installed a custom rom.