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. Its 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.
Ive used the multiple windows, it does take some getting used to.
Im experiencing something different, where a dozen open tabs will disappear. Im suspecting that they get killed when you click a link in mail and it opens safari, but not every time. Then when you go back to safari theyre all gone. Weird.