There is nothing in the laws which says Mike Dean had to stop play because Isaac Hayden was down with a head injury.
It is common practice that the referee will halt play for this sort of injury, though, so you can understand why Newcastle were aggrieved that play continued and Liverpool scored, especially as Diogo Jota was potentially kept onside by Hayden.
Instead Dean let the attacking phase come to a completion and it resulted in a goal. Newcastle may be upset but the rules aren’t on their side. It’s up to the referee.