I also have the same issue. - Only 3 Tabs are visible and there are not in the list to enable them.
What I have tried:
Debugging ionic app with webview
Switched to mobile safari and opened a random page --> still not visible
Restarted safari + mac
Connect and reconnect the phone
Even IOS Simulator has the same issue
Workaround that I found: You can download the Safari Technology Preview, where they available again.