None of my brush modes are functioning

So I’m trying to animate from a tutorial using the fluid brush. I’m pretty new to animate but did flash years ago and work almost daily in after effects.

My problem is I have a shape inside a symbol, and I am trying to add bold, but non-uniform, border on the inside of the shape. Just imagine for the sake of this it’s a circle. Here’s my process:

- I have the circle inside the symbol layer just as a shape
- I click and select the circle
- keeping the circle selected, I then click the fluid brush and change the brush mode to “paint selection”
- I try to paint but it just paints like normal, extending over and outside of the bounds of the shape.

Am I missing something? I've tried it with object mode on and off, with the fill locked and unlocked. Not only that but I tried changing it to EVERY different brush mode and it just paint like normal. That makes me think I’m just doing something wrong. Any help would be greatly appreciated as I can see using this feature A LOT.

Attached video screencapture below

Thanks!