The Farsight Turtle woven echo (reroll LP on uniques) has an area that blocks projectiles in the center. It makes it basically impossible to kill anything in the middle unless you lure them out - which is hard given how small the arena is.
Just noticed this happening with Shuriken’s Blade Shield. Thought it wasn’t triggering at all, but turns out the ring in the middle just blocks them from spawning or removes them when you enter it.