Back then when it was first reported, Apple claimed that this was due to a Core Animation bug that would be addressed in a future update. The good news is that it looks like the problem has since been fixed, according to developer Christian Selig who says they were told by someone at Apple that the problem has now been resolved.
https://twitter.com/ChristianSelig/status/1486847470223118337
This is in the iOS 15.4 beta where it appears that third-party app animations will now be able to take full advantage of the iPhone 13 Pro and Pro Max’s 120Hz ProMotion display. This means that once this update has been pushed out to users, the animation from third-party apps should not be any different from those found in Apple’s own first-party apps, thus creating a more unified experience.
We’re not sure what took Apple this long to fix the problem, but we suppose it’s better late than never. There is still no word on when iOS 15.4 will be released to the public, but as it is available in beta, we are one step closer to its final release.