What if it hadn't been ionic, but was instead the browser itself that rendered the same website differently when in different OS? Oh wait...that's already the case...
Google's browsers don't automatically load your websites with Google's visual branding and Apple's browsers don't load your websites with Apple's visual branding.
There's a battle between some of the large tech companies for appification of the Web at the moment, and that way of thinking is a trap.
You do realize that Ionic is mainly used for App Store and Google Play right? You're free to not use it. It's a correct decision for Ionic to go with iOS and Android standards, from my point of view, and the implementation is superb as always with Ionic components.
A company shouldn't design its website with Google or Apple's visual branding as if it were locked into a closed platform. It also appears to be a problem with things like Flutter, though I haven't looked closely.
There's a battle between some of the large tech companies for appification of the Web at the moment, and that way of thinking is a trap.