If you attempt to edit a prefab in with a Text Mesh Pro UI component attached while in Play Mode, you'll get the following warning:

Prefab Mode in Play Mode was blocked by the script 'TextMeshProUGUI' to prevent the script accidentally affecting Play Mode. See the documentation for [ExecuteInEditMode] and [ExecuteAlways] for info on how to make scripts compatible with Prefab Mode during Play Mode.
UnityEngine.GUIUtility:ProcessEvent(Int32, IntPtr)

1. Install Unity
2. Open the attached project
3. Make sure the Scene view is set to 3D
4. Enter Play Mode
5. Navigate to Assets > LogContent.prefab in the Project Browser
6. Attempt to edit LogContent.prefab in Isolation mode

Expected Result:
The prefab can be edited while in play mode.

Actual Result:
Isolation mode is blocked from opening because of the Text Mesh component attached to the prefab. A warning is shown in the console.

Tested on Windows.

Tested with Text Mesh Pro version 1.3.0

Occurs on 2019.1.0a14, 2019.2.0a1, 2018.3.0f2