Dragon Master -- Bug or Feature !?!??!

[rsolow]

I used some magic items to pump a mage to level Nature 3 so that I could enchant him with Dragon Master before I started summoning Ice Drakes. On later turns when I removed the magic items (and thus my Nature level dropped below 3), the Dragon Master enchantment was lost.

OK.....No problem -- just maintain Nature 3 and the Dragon Master enchantment will stay, right? Well, I really needed to remove that expensive Robe of the Magi that was pumping my mage to Nature 3 so I first equipted a Thistle Mace to pump my mage to Nature 4 and then removed the Robe to drop me back to Nature 3. Well, as soon as I removed the robe, my enchantment went "poof." I experimented a bit and it seems that the removal of any magic item (even one not pumping Nature) will cause the Dragon Master enchantment to disappear.

So, is this a bug or a feature?