IOS – Why iCloud tabs not working iOS 11 iPhone and iOS 10 iPad

From Apple's iCloud Tabs support article (emphasis added):

If you can't access the device that shared the tab, the tab will automatically be cleared from the list after 14 days if the tab isn't updated.

So, any leftover tabs should be gone after two weeks.

This is caused by setting up your iPad from your iPhone’s backup.

In macOS, the file ~/Library/Preferences/ByHost/com.apple.Safari.*.plist holds a key named SyncedTabsDeviceUUID. The value of that string uniquely identifies your browser’s synced iCloud Tabs.

Since I have Safari Technology Preview installed, I have a second file ~/Library/Preferences/ByHost/com.apple.SafariTechnologyPreview.*.plist, with a different SyncedTabsDeviceUUID allowing both Safari and Safari Technology Preview to sync their tabs independently via iCloud.

When you restore a device from backup, that SyncedTabsDeviceUUID gets restored as well, leading to collisions during iCloud Tabs syncing.

If you want to get fancy, you could try backing up your iPad to iTunes, find and edit that file within your iOS backup, then restore it back to your iPad.

Or you could just setup your iPad as new and get a legitimate unique ID for it.