Black cleaver bug?

This is really noticable on fast combo type AD champs.

When you combo two or more physical damage abilities/attacks within one frame, black cleaver passive will be applied once.

The easiest way to reproduce this is to go into Practice tool, buy a tiamat and get W on riven.

Tiamat+W (in that order, plus really fast, almost instantaneously)

Black cleaver will apply only one stack- checkable on the target dummy.

Player expects 2 cleaver stacks. Is this intentional or no?