This bootloader is way too stateful.
So if you try directly with proper vbmeta & system.img, you won't notice anything, it will just work, that's why some people say it works.
If you try system first without doing vbmeta, then it will mark the slot as not bootable. Flashing vbmeta afterwards will NOT mark it as bootable.
it also requires the latest fastboot with --disable-verity and --disable-verification, don't know why yet exactly.
Now if you bootloop to mark it as bootable again, you need to flash the boot partition of the slot.
Even worse than that "fastboot boot" works only if current slot is not marked as not bootable, so any try to launch TWRP fails once your in this mode.
This same issue also applies to users trying to restore to stock sometimes.
This means that the One Plus 6 will require proper documentation on how to do things, and people will have to stick to it, see official instructions in post 2.
Now that we know what the exact cause is, and how to deal with it, let the development begin.
Doesn't seem to be OP fault, just stock Qualcomm bootloader.
Credits to Phhusson and me for making this possible, providing phh a device, documenting the findings and all.