Adding or Removing Avatar Mask on a Timeline Animation Track does not apply until Preview is disabled/re-enabled.
Avatar Mask should take affect as soon as the Mask is added or removed.

repro:
- open attached project
- open Mask scene
- select "Timeline" GO and open Timeline window
- Preview Timeline and notice how the Masks (Humanoid & Generic) are applied... "walk" does NOT play on lower body as expected.
- select Generic Mask overrider track and remove Mask
- do the same for the Humanoid override track
- play timeline...
bug: Masks are still applied... "walk" does NOT play on lower body
expected: Masks are no longer applied... full body "walk" is applied
workaround: Disable, then re-enable Preview mode.

note: Repro steps are also valid when adding a new Mask.