Here is a workaround/solution which works in beta 1 to 3:
Embed each UITableViewController in its own UINavigationController instead of assigning it directly to a tab.
Make the UITableVIewController the rootViewController of the UINavigationController and assign the UINavigationController as the viewController to the tab.
This gets rid of the 'move down' glitch, so possibly the new UITabBarController assumes the usage of UINavigationControllers in the tabs.
BMS
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This bug is still present in iOS18 beta 3.