Nostalrius Begins - Quality wow vanilla realm (1.12)

Hello everyone.

not sure if its a bug in the game, or its as it should be, so here it goes.
Its about the [Elixir of the Sages]

When your class is buffed, with Intellect from a mage, and spirit from a priest.
The effect from [Elixir of the Sages] (18int/spirit) doesnt work,

BUT when you remove your spirit and intellect buff, the potion Elixir of the Sages works?

This is intended behavior. Many elixirs and potions doesnt stack with ordinary buffs. The same goes for scrolls for example.