Yep, I've had something similar happen with rapid rooters.......now I trim about 1/3 of the rooter off and cut a very tiny piece to act as a plug......so far, so far so good......
Fed the Queen tonight and continued on with MC2. I still think there is something to that variable nute thing, especially when using doses under 5Gm. May switch back to MC1 next feed or like your idea, use a high concentration of MC2 to place in feed water, e.g. 100Gms in 100ml of water which would give you a Gm per ml. 5ml in 1 gallon of water would give 5Gm/gal and so on.....