How to reproduce:
1. Create a new project
2. Install the Input System package
3. Create a new Input Actions asset
4. Select the created asset
5. In the Inspector window, click on "Assign as the Project-wide Input Actions"
6. Click on "Edit in Project Settings Window"
7. In the opened window, create a new Action Map
8. Close the Project Settings window
9. Observe the Input Actions asset

Actual result: Newly created Input Actions Map does not get saved as a sub-asset when closing the Project Settings window
Expected result: Newly created Input Actions Map gets saved as a sub-asset when closing the Project Settings window

Reproducible with: 1.18.0 (6000.0.66f2, 6000.0.67f1, 6000.3.7f1, 6000.4.0b7, 6000.5.0a6)
Not reproducible with: 1.17.0 (6000.0.66f2)

Reproducible on: Windows 11
Not reproducible on: No other environments tested

Workaround: Opening the edited Input Action Map in the Input Actions Editor by double clicking the asset and saving it from there