I just watched another Icelandair MAX taking off from Manchester (wasn't quick enough to screenshot it) and it did not have anything flapping about underneath the rear fuselage. So whatever it was that was flapping in the wind under the Icelandair MAX landing at Schiphol earlier today is not normal insofar as it isn't present on other aircraft of type/airline (or any other type).
Bit of a mystery as to what it was hanging and flapping on the aircraft at Schiphol this morning that may never be explained.