Dear Michael,
The machine on which I was seeing this bug recently died. In addition,
the situation had improved and I can't remember whether I had seen the
bug recently before the machine started to decay about six months ago.
If I remember correctly, what did help quite a lot was to empty /tmp
from a recovery shell when the machine was failing to boot. I don't know
whether systemd could do it automatically early in the boot process.
Anyway, I cannot test anymore. Feel free to close this bug.
Kind regards, Thibaut.