Steps to reproduce:
- visit https://en.wikipedia.org/, log in (leave "keep me logged in" unchecked)
- visit https://login.wikimedia.org/, verify you are logged in centrally
- https://en.wikipedia.org/wiki/Special:Userlogin and log in as a different user
- visit https://login.wikimedia.org/ again to check your central session
Actual results: initially, the re-login on enwiki in step 3 will work, but in step 4 I'm logged out on loginwiki, and after that I'm logged out on enwiki as well.
Tested in Chrome 124 in incognito mode (with default setting, ie. no third-party cookie blocking).
Not sure if something changed on our side or in the browser support, but IIRC this used to work.
With "keep me logged in" checked, the local login will remain functional (which makes sense since it isn't dependent on having a valid session anymore) but the central one won't.