iOS 15.1 not displaying some @keyframe trasform:translate animations properly?

I've spoken with users of at least 5 different iOS 15.1 devices, using both Chrome and Safari.

The animation in this codepen link is not behaving properly on any iOS 15.1 devices.

https://codepen.io/kurt-eh/pen/LYzpdKZ

Other mobile devices, including iOS 14.x and Android (and desktop browsers) all display the animation properly.

Is there an issue with 15.1 that would affect website animations?

Animated gif:

Left: display on an iPad running 15.1 (both Chrome and Safari exhibit this behavior).

Right: desktop Chrome in inspect (iPad simulation) mode showing expected animation behavior.