I was not able to find anything in the event list, possibly because the instance was recreated so its ID doesn’t exist anymore? Anyway, I did just create a bug report with as much info as I could, which is not much more than what I already posted in this mail chain. Hopefully we can get somewhere with this : 

https://bugs.launchpad.net/nova/+bug/1911474


Jean-Philippe Méthot
Senior Openstack system administrator
Administrateur système Openstack sénior
PlanetHoster inc.
4414-4416 Louis B Mayer
Laval, QC, H7P 0G1, Canada