Did anyone get a resolution or more info from ESRI on this one? I have portal 10.8.1 and have this issue. Some new users are showing up as "never logged in" even though they are using the portal. Existing users are not having their login date updated even though they are using the portal. There may be some browser specific things going on - our default browser is Edge, and it appears users who click on a link or a portal map / app in an email are not getting the login date updated, whereas those who use Chrome are.
If I go into a user's profile and change something - eg: their units of measure or language settings it updates to what I believe to be their actual last login date, so it appears the info is being provided to ArcGIS Portal, it is just not updating in the last login info.
I am using python to gather usage info from Portal, and the last login info I am getting from the portal via python is also not up to date, so it doesn't appear to be just a user interface issue.
Thanks!