It certainly defied my expectations. Chrome is the same. All I can think is that they wanted an inprivate window's tabs to share a jar and that then gets kinda complicated with popping tabs into their own window, putting them back, flicking between those accidentally while dragging a tab, etc.

Looking at the docs it appears that Safari gets around this by just having a separate jar per tab? On desktop at least, mobile appears to be shared.

Just checked mobile Safari, looks not shared. Open private tab, login to Amazon. Open another private tab, go to Amazon - not signed in.