In Safari every time I close the app or turn off my iPhone, every tab that I have open is deleted. I have absolutely no idea how it started, and it deleted a couple important tabs.
Did you try..
Open Safari Press and hold the back arrow button on the far left hand side of the bottom tool bar. Your browser history menu will appear on the screen.
From here, you can tap on a link to open it.
This post indicates that the problem can be caused by a build-up of stored website data: https://discussions.apple.com/thread/7894860
The recommendation is to clear website data by going to Settings -> Safari -> Advanced (at the bottom) -> Website Data
Then clicking [Remove All Website Data]
The post says that this may have implications for some websites such as having to log in again.