It seems this has been raised as an issue on the official Masto app's iOS, I'd :
https://github.com/mastodon/mastodon-ios/issues/396
Weirdly, it's the other way round on the Android version:
https://github.com/mastodon/mastodon-android/issues/541
The ultimate problem seems to be the official Masto apps aren't fetching all their settings from the server.
I wonder if there are third party apps which allow animations to be set automatically off or on?