I found my new theme behaves strange in iOS. After an inspect, I found this was caused by missing the vendor prefix of some css property.
From caniuse.com I know that the 3d transform property although is supported by Safari and iOS, but it is still needed a vendor prefix to make it work. After add the
-webkit- prefix. the bug
on iOS disappeared. For convenience, I using a grunt task
grunt-autoprefixer to do this automatically.