After some more experience, going through this several more times, I seem to have found a pattern.
Apparently, this kind of problem arises when I reboot the MangoGT without explicitly stopping Mango first.
Having examined the output from systemctl show mango.service -p TimeoutStopUSec which shows TimeoutStopUSec=1min 30s it appears the service file is correctly configured; however, my experience suggests there is a failure somewhere in there. Additionally, a reboot command seems to execute immediately, too soon to properly shut down.
What do you think?