Can't fully edit recorded animation after conversion to clip track.

I lose the ability to add properties to an animation clip if I create it by using record mode in a timeline, then convert the recorded track to a clip track.

Here are the steps:

1. Create a 3D cube.

2. Add a PlayableDirector to the cube.

3. Add an Animator to the cube.

4. Create a Timeline asset.

5. Drop the Timeline asset into the "Playable" field of the PlayableDirector.

6. Select the cube in the scene hierarchy.

7. Open the Timeline window.

8. Add an animation track to the timeline.

9. Drop the cube from the hierarchy into the Animator field on the track you created.

10. Use record mode to record one or more keyframes.

11. Exit record mode and right-click on the track, choosing "Edit in Animation Window."

You will see the Animation window open and the "Add Property" button will be enabled. You can use that button to add more properties to the clip. But...

1. Close the animation window.

2. Right click on the recorded track and choose, "Convert to Clip Track."

3. Right click the clip you just created and choose, "Edit in Animation Window."

You will see the Animation Window open, but the "Add Property" button will now be disabled. You can add new keyframes and change the values of any keyframe. But you can't add new properties to the clip.

Now, a workaround is to create a copy of the cube, create an Animator Controller asset, and drop that asset into the copy's Animator's Controller field. You can then save the scene and drop the clips that are now shown as children of the Timeline asset in the Project window into that Animator Controller (using the Animator window). When you open the Animation window (explicitly, not by using "Edit in Animation Window"), and click on the copy of the cube in the hierarchy, you gain full access to all of the clips you have dropped into the Animator Controller, and can add properties to them in the editor. Your changes have immediate effect and will be visible when you play the timeline.

But why? That is, why do I lose the ability to add properties to a clip recorded in the timeline when I convert its track to a clip track?