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.

"You're browsing it wrong." This and other bizarre behaviours are why you'll never catch me using the thing.