Except that not how it used to work. And that's not how it works for Lazeez who also runs Safari on High Sierra.
Actually, it doesn't.
I totally forgot that I have SOL's tab pinned and whenever I click a link from the forum to an external site it opens in a new tab and when I click the back button, it closes the open tab and switches back to the pinned tab and that tab has been static.
In Safari, if you drag an open tab to the left, all the way, it shrinks and gets pinned and the site is always there. I have tabs pinned for all the important pages (SOL's admin, FS admin, SFS admin, SOL etc...)
I tested the problem on sites that are not pinned and it was unpredictable where Safari scrolls to when you click the back button. And yes, it's a problem with Safari, not the sites.