This happens if you have too much RCR (around 50%, assuming you don’t have any other sources of flat AP cost reduction, e.g. Storm Armor: Power of the Storm).
Taeguk only stacks for channeling skills that cost resource.
With enough RCR and Prism, it’s possible to reduce the cost of Disintegrate to 0, at which point you’ll stop getting Taeguk stacks while Prism is active.
If you try it with lower RCR (remove Crimson’s, your helm gem, and/or some of your gear pieces with RCR to test), you’ll see you still get Taeguk stacks with Prism active.
It’s not a bug, it’s just an interaction between RCR and flat AP cost reduction that makes it possible to reduce the cost of Disintegrate to 0. More specifically, AP cost reduction is applied after RCR. This makes it very powerful with high RCR, and it’s possible to reduce the cost of certain skills to 0.
This isn’t a bug, it’s a logical interaction of the mechanics of RCR, flat AP cost reduction, and Taeguk. Easiest solution: drop a couple RCR rolls off your gear.