"It has Thunderbolt 2 port which is also a Mini DisplayPort 1.2"
Based on the results I have seen on the apple forum, that is a questionable statement from a technical specification point of view. I agree that their "marketing" says it is the same. We believe apple has done something proprietary to that port. I can say that apple users have had more success using the thunderbolt to DP adapter.
Go ahead and test on the HP. If you still have issues, get a monitor exchange.