Dear mantainer, I'm hitting this bug also. Simptoms are exactly the same described by Alban, altough the setup is quite different. The problem arise in a proxmox environment where a PV is configured on a shared iscsi LUN. The iscsi LUN is quite lazy to come online at boot, because the ethernet interface needs about 30 sec to become active. This is due to the spanning tree loop check performed by the cisco switch on the other side. After the boot, multipath -ll return no paths and multipathd goes stuck, as Alban said.
I can confirm that recompiling multipath-tools package with Alban's patch solve the problem. So far, no regression for me. I think would be great to include the patch in a stretch update of the package, the bug could be not so hard to hit in many situations, IMHO. Many thanks to Alban for his analisys and for providing the patch. Thank you, -- Ciao, S.