It is working as intended & is consistent.
As linked above, there is a larger thread which explains it all. The short version is:
The “Defensive buff” of the pierce/bypass abilitiy takes the wording very literally: a defensive buff is if it affects the defence stat in the damage calculation.
When bypass/pierce is successful, it bypasses ALL buffs…
Please see this graphic for an explanation of how the Bypass/ Pierce stuff processes in terms of a logic sequence:
In the example shown (in post 3), you can see a Defence Buff on the enemy heroes (from Freya). This is a Defensive buff & thus can trigger the processing of the bypass ability.