A nymphtear deck uses Sapphire, Amethyst and Obsidian Towers. I've seen the bug occur whenever Nymph's Tears is used on Amethyst Towers, but not with any of the other two types.
Edit: Now in a different game the AI correctly got a Purple Nymph. Hmm.
PS
@Avanger: All AI5 decks carry Quantum Pillars- which produce random nymphs.
Edit2: Stalled one game a bit: First there were two Dark Nymphs normally, then a Red Nymph from an Amethyst Tower, then two more Purple Nymphs normally. So it appears to just happen haphazardly...