[OnePlus 6 ROMs, Kernels, Recoveries, & Other Devel] chrcol:
I see posts from other people with similar issues, its weird.

So here is what I have done now.

I managed to recover the phone out of brick, which puts OOS on slot A, slot B blank.

I discovered TWRP does NOT auto switch every time going into recovery, all the posts saying that are very misleading and can lead down a bad path. But this might be because I use latest TWRP.

One can switch the slot on reboot screen, whilst there is no OS one should NOT try to reboot even to recovery, if you do welcome to fastboot boot loop.

So I did this.

After recovery downloaded 8.x OOS from another site. (this site only has 9.x link).

In TWRP overwrote A slot with it, switched to B slot on reboot screen (didnt reboot), and installed the zip again, then I installed TWRP which goes to both slots.

Tested that both slots boot into OOS, TWRP working on both slots.

Then I installed OOS 9.x the one I got from this site, repeated same process, slot B boots into android, slot A is loading spinning circle forever, although it isnt fastboot boot loop and TWRP still works on it, switching back to slot B got me back into android.

Then I did wipe as instructed in first post, and installed havoc OS zip file 9.x version, in slot B, no errors, tried to switch to slot A and suddenly a message cannot mount system, no permission. So the wipe and installing havoc OS broke the other slot.

If I try to reboot in slot A its now fastboot boot loop. I am now trapped in slot A again unless I find way to switch boot slot without doing the recovery app.

After so many test runs what I am clear about right now is that the instructions in the OP are simply wrong. I dont know why the havoc OS installer seems to completely break the slot that you not installing to either.

So from what I can see the only way this may work is with one slot broken, and hope you never accidentally boot into that slot, else you soft bricked. In the past I have had niggling issues to get phones working on custom rom when I first get them, but never had this kind of trouble.

I am about to look to see if there is a fastboot command to switch slots, but if no such command exists then it looks I am back to recovery, and also on recovery all internal data is wiped so if people get in this position and they have nandroid backups, then those get lost unless you copied them off the device beforehand.

So the good news is the newest fastboot binary can change slots.

I am out of the fastboot boot loop without the recovery app, I ran these commands.

K:\ANDROID\ONEPLUSSIX>fastboot --set-active=b
Setting current slot to 'b' OKAY [ 0.027s]
Finished. Total time: 0.029s

K:\ANDROID\ONEPLUSSIX>fastboot reboot recovery
rebooting into recovery OKAY [ 0.000s]
Finished. Total time: 0.002s

Instead of lumping me in recovery it has rebooted the system, although I installed havoc os onto slot B its simply booted OOS again. This is what happened right at the start, havoc os install zip reported success, but had one broken slot, and working slot still had OOS.

Also I checked the youtube video but it was fastfowarded past the install screen so I dont know if my twrp install log is identical to a successful install, But on slot B there was no errors, just step 1 and step 2 followed by done. Attempting to install on slot A will print a red line of unable to mount /system before the done. But slot B has no error. Slot A boot loops on fastboot, slot B boots OOS. I am about to test if I can reflash OOS on slot A.

I successfully reinstalled OOS on slot A, no mount error shows up and it removes the boot loop on slot A as well. Right now it simply seems as if something is wrong with havoc OS installer, or an extra step is required I am missing.