GrowthExperiments redirects (most) users to Special:WelcomeSurvey after signup. CentralAuth redirects users to the central login wiki after login (also after the implicit login that happens during signup), to establish a central session. With the current implemntation, this two might conflict, and thus GrowthExperiments might interfere with central login (up until the user does a non-signup login, which would fix the central session).
We'll change the hooks used for redirection in GrowthExperiments.
Originally this was investigated in the context of T267273: [arwiki] Submitting a POST on a form redirected to immediately after account creation sometimes logs user out ; since this issue is about central login failures, not local ones, it cannot be the cause, but in theory the central session would be used automatically to recover a lost local session, so this issue might be a secondary cause of that bug by preventing the fallback mechanism from working. We won't know for sue until this change is in production, unfortunately.