So it turns out that the Mad Alchemist's Potion was bugged after all, and that the recent patch to the PTR took care of the issue. Upon logging in and downing the 4 potions I had on me, I received 4 consecutive and different buffs, having to had canceled the previous one, of course.
I also looked into answering a question I had upon first hearing of this potion. The description always stated that it would not replace an existing elixir if one was active. However, I wondered if this was Guardian and Battle Elixir specific. After taking two pots back-to-back without canceling the first buff, that does indeed appear to be the case.
The question now becomes whether or not the pots will always apply a complimenting buff if the player already has a single elixir active. For example, if I had an Adept's Elixir (a Battle Elixir) active and I consumed a Mad Alchemist's Potion, would it only try to apply a Guardian Elixir when it procced? Or would it try to apply both types and have a greater chance at failing as a result?
After a bit of testing, it seems that the Mad Alchemist's Potion DOES recognize Elixir types, and tries to apply it's compliment as a result. In six of seven attempts where an Elixir was already active, the potion applied the compliment. So, one can argue that the potion does indeed recognize elixir buffs that are already active on the player and automatically attempts to apply the other elixir class. However, given that there were only seven trials, the observations could very well have been due to chance, and should be taken with a grain of salt. We'd need to complete many more attempts before we can come to any real conclusion on the matter.
Still, given this limitation, today's fix to the proc-rate of the potion itself brings it up to the level it should be. WAI, this is definitely a worthy pot all Alchemists should consider.