Since it had broken in the last upgrade on my Pinephone Pro, I reinstalled beta15-rc4, and then upgraded after making the change to pacman.conf as described above. This failed, and I ended up again with a broken system. However, I realised that I had neglected to switch the branch to arm-testing.
I noticed the note that the issue was fixed in the latest update. So, I reinstalled Manjaro, switched to the testing branch, and upgraded (without changing pacman.conf). This failed.
It seems I should simply wait before attempting to reinstall and upgrade again.
The issue is the same as it was previously (see link).