Issue was caused by $wgCookieSameSite= "Strict"

I have tried but the issue remains.

In T289780#7312022, @Krinkle wrote:

If Varnish is configured to cache-miss on mwuser-sessionId then it is incorrectly configured. This cookie is not in any way related to the server-side of MediaWiki or the concept of MediaWiki backend sessions.

See WMF's Varnish configuration as example:

		if (req.http.Cookie) {
			set req.http.X-Orig-Cookie = req.http.Cookie;
			if (req.http.Cookie ~ "([sS]ession|Token)=") {
				set req.http.Cookie = "Token=1";
			} else {
				unset req.http.Cookie;
			}
		}

The MediaWiki PHP session cookies are named as <dbname>Session with a capital S and ending in the word "Session". If the word "session" is matched more arbitrarily, then it would also match numerous unrelated JavaScript cookies relating to statistics and performance counters.