I'm not able to reproduce this. With default settings, selecting the Timeline opens by itself. Selecting Measurement Log opens in the same tab group as Timeline. If I select "Close" with the Measurement Log panel visible it closes just the Measurement Log. If both the Measurement Log and Timeline are open at the same time and and choose "Close Tab Group" it closed both.
Hi Jeffrey Thanks for trying. Given your feedback I tried resetting preferences. After restarting , in the Essentials workspace I started a new document. I Opened the timeline and no measurement log. So I closed the timeline and opened the measurement log. The timeline opened with it ! Now both open and close together. Windows 10 1903 Photoshop v21.0.0 GPU Nvidia GTX1080