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Put your iPad in horizontal way, then open Safari. In Safari, on the right side or left section, you will see a small x button under the address bar (in left side). Tap it to close that window and put the other window to full screen. If you want to put it back to split screen, then just tap and hold your finger on the tabs (two boxes icon) at top right and select Open in Split View)
If that didn't work, then close Safari. Go to Settings > Safari and tap Clear History and Website Data, Only do this if the above doesn't work.
See this video ---> https://www.youtube.com/watch?v=WE0p67QyRMA&t=37s
Please let me know if that didn't work, so I will continue to help you until we fix the problem
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