I'm back on Oreo now having tried the sideloaded pie. I still have the option to join the beta in members. Are you sure you're on the latest Oreo after flashing back?
I'm using a ee locked s9. I flashed btu firmware then installed beta via recovery sideload then booted into beta install Samsung members, note banner is present, register for beta. Check notices to see if beta notice is present dont open, re flash btu or xeu firmware. Boot into stock oreo firmware and download Samsung members. Open app go to beta banner and check if you have the option to deregister if yes dont click if not click register then go to notices and check if beta notice is their then open notice. Now go to software update and install beta. Restart then go back on to Samsung members click notice then open Hotfix notice then go to software update and install hotfix. Hit restart and enjoy
You keep saying register your phone in Samsung Member app. Which you don't need to.
My galaxy s9 didn't even register it didn't let me. You just need to have BTU or XEU firmware and open Members app and click on notice then you should be able to sign up for the beta. That's what I did