Originally reported by Nirzar.
Visit https://hi.m.wikipedia.org/wiki/%E0%A4%AC%E0%A4%BF%E0%A4%B2%E0%A5%8D%E0%A4%B2%E0%A5%80?mobileaction=beta#/languages and you'll see the old language overlay.
The configuration says that the structured language overlay needs to load:
Running the following code in the console results in "A", which is what is expected:
mw.experiments.getBucket( mw.config.get( 'wgMFExperiments' ).languageOverlayBeta, mw.user.sessionId() )
So the bucket is A, and A bucket must see the structured overlay, but somehow it doesn't.