For mobile Safari on iOS/iPad, the back button imo is just completely broken. It’s either a bug, or Apple might say I’m ‘holding it wrong’. One version it just stopped doing its one job correctly and it’s messing with my mental model of how I arrived at each tab. Currently:

Safari iOS: Be on a page, tap hold a link, click Open in new tab, go to new tab. The Back button should be grayed out and isn’t, and clicking it closes the tab. (???)

Chrome iOS: Be on a page, tap hold a link, click Open in new tab, go to new tab. Back button correctly grayed out as the tab has nowhere to go back to.