I dont know if its SUPPOSED to be like this, but if it is, i would personally suggest it changed.
When im playing say Mundo, and turn on burning agony, or gem knight, and using His ult. it channels as normal and uses up Hp/mp respectively. When you get silenced when those are on, you cant turn them OFF. And some hero combinations can keep you silenced for well over 10 seconds. So, for example. gem knight. 10 seconds of silenced, and probably CCed at the same time. his entire mana pool is gone.
Mundo, silenced for 10 seconds. Probably at least 500 health, probably more.
Just seems something that can be abused, or just incredibly annoying. So i dont know if its a bug or not, but personally i think it would make much more sense that at least you could turn them OFF when silenced