looks like the files used for it was changed however the ratio in the portal source wasn't adjusted to match the new file making it appears stretched and oval-shaped
https://meta.wikimedia.org/wiki/File:MediaWiki-logo_sister_1x.png
https://meta.wikimedia.org/wiki/File:MediaWiki-logo_sister_1.5x.png
https://meta.wikimedia.org/wiki/File:MediaWiki-logo_sister_2x.png