Also www.reddit.com is/was doing the same back button hijacking. From google.com visiting a post, then clicking back and you would find yourself on Reddit general feed instead of back to Google.
News sites are doing it too. Displaying a full display ad when you try to leave
I do not see this behaviour on the latest version of Firefox. I do use old.reddit, however.
I usually find the back button just doesn't work on new Reddit at all.
IIRC Reddit is also doing the same thing on their mobile (Android) app.
I'm pretty sure what you're describing is this long-standing bug[1] I've experienced only when using Mobile Safari on Reddit - affecting both old.reddit.com and the (horrible) modern Reddit. It just doesn't happen in other browsers/engines except on iOS. It's especially annoying on an iPad when I tend to use back/forward instead of open-in-new-tab-then-close on iPhone.
[1] At least, I hope it's a bug.