Please set $wgBlockDisablesLogin false for wikimediafoundation.org.
In https://noc.wikimedia.org/conf/highlight.php?file=InitialiseSettings.php it is currently set to:
'foundationwiki' => 'true', // RT #690
which can just be deleted.
It makes no sense to have this enabled since wmf.org is a public wiki and it only prevents them from editing their talk page while blocked, which is not a concern.
There was also some discussion at https://wikimediafoundation.org/?oldid=87432 about disabling the setting.
I will ask Guillom and Philippe to comment here as well.