So far I highly suspect this commit[1] to be the possible offending one and it would "fit" with the bionic systemd version in Ubuntu[2] vs upstream introduction of the change :
$ git describe --contains 006aabbd05
v237~47^2~2
[1] 006aabbd0 mount: mountinfo event is supposed to always arrive before SIGCHLD
So far I highly suspect this commit[1] to be the possible offending one and it would "fit" with the bionic systemd version in Ubuntu[2] vs upstream introduction of the change :
$ git describe --contains 006aabbd05
v237~47^2~2
[1] 006aabbd0 mount: mountinfo event is supposed to always arrive before SIGCHLD
[2] rmadison
systemd | 237-3ubuntu10 | bionic | source, amd64, arm64, armhf, i386, ppc64el, s390x
systemd | 237-3ubuntu10.3 | bionic-updates | source, amd64, arm64, armhf, i386, ppc64el, s390x