docker-machine looks for a version number in the boot2docker ISO header. See here:
https://github.com/docker/machine/blob/master/libmachine/mcnutils/b2d.go#L310

v18.09.0 does not appear to be embedding a version number in the same way as previously.
Offset 0x8028 in v18.09.0 ISO has the string Boot2Docker
Offset 0x8028 in v18.06.1-ce ISO has the string b2d-v18.06.1-ce

The consequence of this is that docker-machine repeatedly logs
Unable to get the local Boot2Docker ISO version: Did not find prefix \"-v\" in version string"
Default Boot2Docker ISO is out-of-date, downloading the latest release...

And, more seriously, it does re-download the boot2docker ISO for every machine created!