To reproduce:
1. Open users attached project "VisualElementNotOnSecondInspector.zip"
2. Add another Inspector window so both inspector windows can be seen at the same time
3. Select "DirectorTimeline" timeline from the project folder
4. In Timeline window select Animation Track
5. Observe in both inspector windows you can see two options, to enable Apply Avatar Mask and avatar mask input field
6. Select the marker in the Timeline window
7. Observe both inspector windows

Expected result: in both inspector windows you can see a public string input field
Actual result: only in the original inspector window a public string input field can be seen

Reproduced on:
2019.1.0b2 2019.1.2f1 2019.2.0b1 2019.3.0a2

Not reproduced on:
2019.1.0b1