I build a app Demo, use hidesBottomBarWhenPushed hide Tabbar in Push Animation.
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Edit: After the release of 12.1.1, this issue has been fixed. You can keep the original structure.
If you change your structure from
UITabBarController -> UINavigationController -> UIViewController
to
UINavigationController -> UITabBarController -> UIViewController
you will find this issue has been resolved. I really don't know why Apple doesn't fix this issue.
In iOS 12.1, this problem becomes more serious. You can see the TabBar text jump above the TabBar every time, if you use gesture to pop back.
Note: This way can definitely solve this problem, but I am not sure whether it's a good idea. Also, if your structure is quite complicated, you need to change lots of stuff.