Currently, if an Element features an Opacity Mask composed of a Linear Gradient, if this Opacity Mask is keyframed in any animation, the direction of the Linear Gradient featured within can no longer be reversed in any context (inside or outside of the Storyboard).

See attached video.

Repro Steps:

1) Create a Studio Project featuring a MainPage.xaml

2) Add a Background colour of your choice to the RootGrid.

3) Add an OpacityMask to the RootGrid, of a LinearGradient type.

4) Navigate to the Animation/Storyboards Panel, and create a new Storyboard named "MyStoryboard".

5) In the 'MyStoryboard' Timeline, select the RootGrid, hover over its 'OpacityMask' Property's 3-Dot Menu, and select "Create Keyframe".

6) Still within the 'MyStoryboard' Timeline, select the RootGrid now named "Grid0", and click on its OpacityMask Property to expose its Brush Panel.

7) With the Brush Panel open tot he LinearGradient Tab, click on the 'Reverse the gradient stops' icon directly to the left of the Gradient Stripe.

Observe how no reversal takes effect.

Expected Results: Since you can manually 'reverse' a Gradient in an animation my moving the Gradient Stops around, it would be expected that the 'Reverse the gradient stops' function would work.
However, if this is a restriction of the framework, then this function button should be greyed-out/disabled in this specific context.