I have independent confirmation from @pfaffman (thank you) that he’s seeing this behaviour on mail sent via SendGrid:
List-Unsubscribe:
=?us-ascii?Q?=3Chttps=3A=2F=2Fforum=2Econtoso=2Ecom=2Femail=2Funsubscribe=2Fb87c085a68d9210e78?=
=?us-ascii?Q?6478e2a54849e5bca9b8002ff0081b0a77466e0?=
=?us-ascii?Q?f71c27=3E?=
List-Unsubscribe-Post: List-Unsubscribe=One-Click
and that the Unsubscribe easy link is not present (which is bad).