Thanks Nick for pointing me to this bug; I was working on the same issue, as reported in a duplicate bug.
As far as the classic desktop is concerned, this bug occurs not because of the failed mount, but because of "unshare" failing (see the discussion in bug 1971955); in any case, the analysis by Jiwei Sun is correct: it's happening because of the added SystemCallFilter entry to the udev unit file.
Thanks Nick for pointing me to this bug; I was working on the same issue, as reported in a duplicate bug.
As far as the classic desktop is concerned, this bug occurs not because of the failed mount, but because of "unshare" failing (see the discussion in bug 1971955); in any case, the analysis by Jiwei Sun is correct: it's happening because of the added SystemCallFilter entry to the udev unit file.