Firstly, why not?I stand by my assertion that Druids should not be frontlines.
Secondly, Circle of the Moon (and circle of spores) begs to differ.
Thirdly, it take more than an of AC 19 to make a character a front liner.
I would fix this by granting an AC bonus while the Halo of Spores is active.
I would fix it by putting an Ankheg breastplate in the character's path early on. Because if you can fix something with changing the rules it make the whole game-world more robust.