So it was a multi-step process, but it's actually pretty simple. LOS has a good guide here: https://wiki.lineageos.org/devices/fajita/fw_update
I had some trouble figuring out how to extract the Rom using the Payload-Dumper-Go program, but this is what I did running Windows 10:
1. Copy original image (zip archive or payload.bin) to the same directory as payload-dumper-go exists - make sure you have the latest one, otherwise you'll continue to have problems (I mistakenly used the next-most-recent img and had to go through everything a second time)
2. in Powershell, enter "./payload-dumper-go payload.bin"
3. The extraction took a little while (maybe 10-15 mins, I wasn't really paying attention).
4. Boot into Fastboot and using the Command prompt enter the 20 +/- commands on the LOS guide.
Once everything is extracted, flashing the files took about 5 minutes.
Hope that helps.
Thanks for posting this issue. I have noticed this issue as well lately and was wondering how to fix it.
Just to make sure, if I used OnePlus6TOxygen_34_OTA_046 zip file to install originally, i should use Payload-Dumper against it to extract the files, correct? Or should I use the newest available OP6Oxygen OTA update?
Looks like the command use the slot=all argument, but usually when i try to use it, it doesnt work, stating that it cant find the partition. Does this need to be applied to both slot A and slot B?