Safari clears all tabs when closed.

You stated in your original post "fully close the app by double clicking". After you double click are you also swiping the open Safari window up and off the screen?

Force an app to close on your iPhone, iPad, or iPod touch - Apple Support

Have you checked to see if you are in Private Browsing mode?

Turn Private Browsing on or off on your iPhone, iPad, or iPod touch - Apple Support

Are you using iCloud Tabs?

Set up and use iCloud Tabs - Apple Support

When you open Safari now is it showing something like this?

I've had this problem too and I've solved it yesterday. It has to do with having too much stored website data (not cache or cookies or history).

Go to Settings -> Safari -> Advanced (at the bottom) -> Website Data.

Here, there was over 32MB of data on my phone, one site even storing 17MB of data on the mobile browser. I've deleted all of it using the function at the bottom. Et voila, problem disappeared, my tabs now always stay saved in Safari. I had to log in again on some web sites but that's a small price to pay. I haven't run into any other issues yet so I can highly recommend this. Hope it also works for you!

With no changes to settings and all of the above information I would honestly have to suggest that you contact Apple Support directly as it may be a new issue. Contact - Official Apple Support

You can also provide feedback regarding this here. http://www.apple.com/feedback/

I hope they are able to get this resolved for you and I apologize for my own lack of resolution. If you do come to a resolution please share in this post as it will most definitely help others. 🙂

Send Apple feedback. They won't answer, but at least will know there is a problem or a suggestion for change. If enough people send feedback, it may get the problem/suggested change solved sooner.

Feedback

Double clicking the Home button and swipe Safari upwards. Go to Settings/Safari and clear History and Website Data. Open Safari and test.

Safari - Clear the history and cookies on your iPhone, iPad, or iPod touch

Safari website data on your iPhone or iPad - How to clear

This has been happening to me as well, starting the evening of March 19. Every time I close safari, it clears all of my open tabs, and the "recently closed tabs" are completely empty. I am also on the most recent version of iOS. This has never happened before but has happened consistently for the past 24 hours. Extremely frustrating.

Sadly, I am experiencing the exact scenario as you, all of which happened just before I updated the newest iOS and after updating! It's so frustrating when the tabs where always there for me to refer and now they just go blank. Even though it can be found in History, I've to go through a list...

Sure hope Apple can get this resolved asap! Good luck to us.

I also have had the same problem beginning at some point yesterday. I am very confused as to why. I wish i could help but i wanted to ask something: Has your device been running out of storage, deleting and restoring your photos or runnin slowly? I know mine has so I was wondering if its connected in a way.

Is your iPhone running the same version of iOS as your iPad? [Settings > General > About > Version]

Double click the Home button and swipe Safari upwards. Go to Settings/Safari and clear History and Website Data. Open Safari and test.

And I've just done as per your suggestion! :) Hope it'll be resolved soon!

Ive been dealing with this same issue fir months and this tip solved the problem. Agreed, having to sign into a few websites again is a small consolation. Thanks for sharing your advice!

Seriously thank you so much! Ever since I updated to iOS 1.1.1 my phone always closed tabs when I swiped it up. Cleared out website data, swiped up safari, and it’s now fixed!

Thank you!