SFT (Pax Exam, really) sometimes gets "stuck" - only if there are other Karaf instances running/waiting to shut down?

see https://lists.opendaylight.org/pipermail/odlparent-dev/2018-January/001681.html

I've just, totally by coincidence, run into something similar locally, here's what happened:

I had, earlier today, started a Karaf, and then attempted to shut it down but either forgot to press enter, or it got stuck there, I found it open hours later in my terminal:

karaf@root()> shutdown
Confirm: halt instance root (yes/no): yes 

a build got stuck in SFT for 1/2h like this... and when I hit Enter for the other Karaf, then SFT unblocked and that build finished.

Attached are the jstack stack traces; Launcher.jstack is mvn (not interesting), surefirebooter.jstack is the forked JVM running Pax Exam (not very interesting) and Main-18876.jstack is my first Karaf from manual with held up shutdown and Main-26448.jstack is the Karaf launched by SFT that got stuck..