I have a service that relies on a samesite=none cookie. I have seen that this cookie is not sent for about 1% of users in a cross domain context. This is the exact case my site needs samesite=none, for a basic form SAML POST login from another IDP -- login fails without it. The cookie in question also has the secure flag. I'm wondering if there are any known browser addon's or other reason the cookie may not be working for some users? I found https://bugs.webkit.org/show_bug.cgi?id=198181 but I dont think thats the only cause -- the user agents are spread out more. I did notice the incidence is very heavily skewed to desktop users, with few mobile. Also, those who experience this, seem to experience it consistently.
Tried various chrome settings, installing a bunch of the top browser addons to see if they produce this behaviour, to no avail. Looking at workarounds now, but seek a better understanding of why this is happening to some particular users.