While Taliyah is casting her knock up, until the casting animation is finished and the rock has popped out you are unable to cast any other spells. Her W isn't described as a channel ability and you can still move freely while casting it.
I have tested this and was able to frequently if not consistently replicate the bug in a custom game.
I was able to replicate the bug by casting her W, and attempting to cast another ability immediatly after.
This isn't a bug. It's a very deliberate part of her design, so that you can't cast E once you're almost certain your W cast will hit. Apparently Riot didn't want a medium-ranged direction-of-your-choice knockback to have additional damage on top of it as a near guarantee.
It also kinda makes sense thematically. It's more logical for her to focus on one earth-bending trick at a time rather than attempting multiple very different moves simultaneously.
This has been known since PBE release. I dont know if it is honestly intended, but I would have expected it to have been fixed if they were going to, ... or maybe it has such a low priority at the moment that we will just have to tough it out.
