Have you had a problem with open tabs in Safari (iPad Pro) disappearing? Very strange...
For a couple days until I realized I was dragging them and they were becoming separate windows of Safari. You can drag up from the bottom of the home screen and find several instances of Safari windows running. Those are your missing tabs. It’s about the new multi-tasking. You can arrange them to be opened side by side. Great for cutting and pasting between webpages. Takes some getting used to.