3 years later... still broken... siphon doesnt effect Spider's turn meter at all. I thought he just wasn't using it on auto so I switched to manual and directly targeted the large spider with siphon... the turn meter which had been just below half actually went up... fix the damn bug.
This sounds to me like it failed the accuracy check if it's happening with consistency because on Normal spider this absolutely works as I just tested it.
However, it won't work on Hard spider because the boss is immune to turn meter decreasing effects.