How to reproduce:
1. Open the attached "IN-139735_IconBug.zip" project
2. Select the "Assets/IconBugTest/" folder in the Project window
3. Export the folder as a package (Right-click on the folder → Export Package → Export)
4. Delete the original "IconBugTest" folder
5. Import the exported .unitypackage file back into the Assets
6. Observe the Overlay Menu in Scene view

Actual result: The "Test Overlay" button in the Overlay Menu has "TO" as its icon
Expected result: The "Test Overlay" button in the Overlay Menu has a green circle as its icon

Reproducible with: 2023.3.0b10, 6000.0.73f1, 6000.3.13f1, 6000.4.3f1, 6000.5.0b4, 6000.6.0a3
Could not test with: 2023.2.0a1 (The test overlay bar does not appear)

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

Note: The green circle icon appears as expected after restarting the Editor