It drains from a random quanta pool (which has more than 0 quanta in it). It doesn't matter how much is in the quanta pool - if you have 75 light, 30 aether, and 1 water, there's 1/3 chance for each. Devourers hit splashed elements really hard.
edit: that said, two devs completely shutting down one element in a trio is rather bad luck.