@andorinhariver Okay, I've tinkered a bit with my device and I don't think this is a UT related issue.

I'm not sure really why, but the Oneplus 6 seems to just generally have a lot of trouble trying to boot to other OSes.

You just have to try and try and try over and over again.. would be helpful if there were logs of any kind showing what's wrong. Bafflingly, going into the recovery mode works just fine.

Same here. It worked on first boot, and now it's just stuck on the unlocked bootloader warning.
I can boot into Halium recovery, but from there I still can't boot into UT.
If I reinstall, it gets up to the stage where it has the animated Halium(?, looks like LineageOS) logo and then reboots. After that it should reboot into UT, but it just reboots to the warning and stays there indefinitely.