Steps to reproduce:

Open the attached project
Open the Console window and clear it
Select "PB-9 > 1. Open the repro scene" from the menu bar
Select "PB-9 > 2. Rename one label in each document" from the menu bar
Select "PB-9 > 3. Close the scene (click Save in the prompt)" from the menu bar
In the "UI Assets Have Been Modified" pop-up, click "Save"
Observe the Console

Actual results: The edits that were made in Step 4, which should've been saved in Step 5, are lost. Repeating Step 3 and expanding all documents shows that the edits were not saved

Expected results: "Save" writes every listed document, as "File > Save" does with the same eight edits ("PB-9 > 4. Control: same edits, then File > Save": all eight saved)

Reproducible with versions: 6000.7.0a6, 6000.7.0b3, 7000.0.0a1

Can't test with versions: 6000.0.79f1, 6000.7.0a5 (Main Stage UI authoring is not available)

Tested on (OS): macOS 26.6.2 (Apple Silicon)

Notes:

Silent data loss - if a user edits a document and a template it uses in the Main Stage, then opens another scene and clicks Save, changes are lost. The scene-close pop-up has no Cancel button