To reproduce:

1. Download attached project “MenuItemTest.zip" and open in Unity
2. Close "Bolt" window when it popups
3. Press Window and observe that there is "Graph" menu item
4. In Project view find "GraphWindow.cs" and open it
5. When .cs file opens replace "[MenuItem("Window/Graph", priority = 0)]" with "[MenuItem("Window/Bolt/Graph", priority = 0)]"
6. Save .cs file and come back to Unity
7. Wait until it recompiles
8. Close "Bolt" window when it popups
9. Press Window and observe that there is no Bolt menu item

Expected results: Observe that there is Bolt > Graph menu item
Actual results: Observe that there is no Bolt menu item

Notes:
- This issue appears only on macOS
- On Windows everything works as expected - You can see in Bolt > Graph menu item in Window tab
- Tried to create a small repro project with "[MenuItem("Window/TEST/Do Something")]" but it was unsuccessful - everything works as Expected on macOS
- The Window menu in macOS seems to be the only one affected - in Tools, nested items are accepted
- Could not check on Unity 2017.4, 2018.2, 2019.1 and 2019.2 because project becomes corrupted

Reproduced on Unity 2018.3.1f1 and 2018.3.3f1