The same bug is in Chrome, though they probably don’t call it a bug.
It arguably made sense not to cache HTTPS content in 2000. Not in 2015. Not caching HTTPS means that when you go back, the page loads afresh from the current URL.
Which means when you go back to look at something, there’s a good chance it won’t be there again.
This is terrible, terrible behavior, anti-user and violates all sorts of usability guidelines.
I used to use Opera because it was the only browser that implemented caching correctly.
When I hit the back button, I FUCKING WANT GO BACK AT WHAT I WAS LOOKING AT BEFORE DAMN.