It's fairly easy to test it for yourself, using mobs that give you the "seems to have high defense" message, or no such message.
In theory, it works like this: Cast Dia, and it'll likely downgrade that message. Either it will go from "high defense" to no message, or from no message to "low defense." Then, cast Bio. It should have its original message back, meaning the Dia effects have been cancelled. Find another mob, and cast Bio first this time...and watch the message remain the same, since Bio can neither stack with nor be overwritten by Dia. Run through Dia II and Bio II as well, and as every test I've ever seen documented in the forums clearly shows:
Dia > Bio > Dia II > Bio II
This seems to be the hierarchy of what cancels out what, moving from right to left. I don't know about this changing in any patch, but I certainly don't recall reading any patch notes about it. I'm pretty sure it's the way it's always been. In any case, it should be simple enough to check.