My dads iPhone 6 Plus has had booking.com app installed on it since the release of the iPhone 6S Plus. Neither one of us have ever deleted this app; granted I use it more often than my dad as I book reservations in Niagara Falls frequently. I also believe that any app updates are updated faster on my device than his..
So to further clarify; are you suggesting that the app in his iPhone did not have this buggy (app update) version installed; and mine did.?.
Does using the actual app have anything to do with it.?.
Honestly, I am not completely convinced that booking.com app is the only culprit here from my own investigation; although I hope that I am wrong. You don't hear me say that very often, but it is very true in this case. Just want a working fix asap (like everyone else).