Kayn ult vs Black Shield

So a friend and I just tested this interaction with Kayn and what we found is that if Morgana has black shield on a target before Kayn lands an attack Kayn will not be able to use his ult even after landing attack as long as Black Shield last, however, if Morgana is hit first and then Black Shields Kayn is able to use his ult normally.

Is this the intended interaction and why does it work this way? Without it being a CC shouldn't Kayn be able to ult Morgana regardless of Black Shield?