The bug is fixed in iOS 15.4 beta, confirms Apollo Reddit developer @ChristianSelig.
Yay, got a little message from someone at Apple today that this is fixed in the iOS 15.4 betas and all apps automatically get 120 Hz animations on supported iPhones going forward, and just confirmed myself. (See attached, 1/120 = 8.33 ms)
Other improvements in iOS 15.4 beta include Face ID with a face mask, new emoji, iCloud keychain notes, a new Apple Card widget, and more.
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