There are previous allegations and there are current.
UPMC ER doctors likely questioned the injuries and continued questioning as their reports didn’t align with what was being reported by the family. The parents are aware of what abuse questioning looks like from prior incidents so even if they were not directly told CYF (Allegheny county CPS)/childline/police would be called, the parents were probably already suspicious.
UPMC (I’m in area and field, I do not have details regarding this case) typically does not leave a child alone with the parents once abuse/imminent danger has been considered though so I’m wondering what occurred when they actually left the hospital.