It is a bug in the upgrade script that messes the GRUB installation. Possibly, only on UEFI firmware (my case).
The bug I refer to is much "older" than that. I'm not sure but could go back to LiLi 3.x.
100% sure though that its was not 5.x, Upgrade or UEFI related since it was on last version 4 last I got it and in legacy.
Educated guess that it would be audio chipset brand / driver and or kernel related.
I will pay more attention next time I come accross the problem since I come across a system doing it from time to time.
Bonus: I think, pulseaudio --kill also solved it.