TWRP is also made for A/B devices. When it is installed, it explicitly says that it is being installed on both slots (something like “Patching slot A” and then “Patching slot B”).
I actually did install it two weeks ago, as written in my previous post:
However, I confirm that even with the latest official version of TWRP (3.3.1-2 of July 2019), OTA does not work for me: same identical issue.
This is unfortunately not an option for me: this is my only phone that I use on a daily basis and backing up, restoring, re-configuring everything the way I need it would take too much time and is too risk prone. I don’t use any external account for saving apps and data, so the whole procedure would have to be manual.
I will therefore keep on updating manually from recovery, at least for the time being.
I am now wondering if this issue with OTA not working is only affecting me or if it is something affecting all users of OnePlus 6 (enchilada)?