In the documentation Configure port-forwarding | Yunohost Documentation the mentionned command is “firewall restart” so maybe the activation is not on start, this might an indication that you have found a real bug.
I detect that UPnp is not enable after a reboot because :
A) Yunohost complain by email to the administrator that many services are not accessible from the Internet.
B) The webUI display "UPnP is disable "
C) The “Diagnostic” command of the WebUI complain that a lot of Internet ports are not accessible from the Internet.
Mine is also disabled after a reboot, but I guess that it’s the default behaviour. Though I don’t know the motivation about it. And I don’t care, as my system works fine without uPnP.
UPnP support has never really been really maintained unfortunately, the code is a mess (and yes, the current code will disable UPnP forever if it fails even once), it’s hell to test, it’s not clear what fraction of home routers support it x_x