Currently, if within a Timeline, any valid manually-pasted value into a Property field, will instantly key a keyframe with its value on the Timeline.

However, if the value is pasted using Studio's built-in Property Copy-Paste functions, it is not recognised as a value to key.

Repro Steps:

1) Create a Studio Project featuring a MainPage.xaml

2) Introduce a Rectangle into the Page's root Grid, set its dimensions to 512x512, and give it a LinearGradientBrush 'Fill' of any kind.

3) Navigate to the Animation/Storyboard Panel, and create a new Storyboard
This will engage 'Record Mode' for this new Storyboard.

4) With the Timeline open, select the Rectangle introduced in Step 2), then hover over its 'Fill' Property, and from its 3-Dot menu, select 'Copy', to copy the values of the Property.

5) Being careful to not copy anything else into the clipboard, open the Fill Property, and change the Gradient Stops to different colours and/or positions.
This will place new keyframes on the Timeline.

6) Move the Playhead in the Timeline a little bit forward to a point where there are no Keyframes keyed yet.

7) With the Rectangle Element still selected, hover over its 'Fill" Property, and from its 3-Dot menu, select 'Paste.

Observe how the Property Value has not been successfully pasted into the Property field.
Observe how a Keyframe was note keyed on the Timeline at the Playhead position.

Expected Results: Values can be copy-pasted in the context of a Timeline.
Expected Results: Whilst a Timeline is open and recording, Property values can be keyed by copy-pasting new values for them in their Properties.