It is apparently a login update with WordPress. An “improved” control mechanism during the login process to check if a browser can store cookies. This cookie, however, is only set in the wp-login.php file and in many login scenarios, the cookie is set too late to be of any real use.
Luckily for me, my membership plugin provided an update to work around the issue. As for solving directly, I unfortunately can’t provide much assistance but hope this information helps!