Reproduction steps:
1. Open the attached project
2. In the Project window select “Skeletal_file_for_unity” and make sure that in the Inspector window Materials tab settings are the same as in the attached image “settings.png” 
3. If Apply button is greyed out export the textures with different settings, then delete the files, and repeat with said settings.
4. Observe the Skeletal_file_for_unity.fbm folder

Expected result: Textures are extracted
Actual result: Textures are not extracted

Reproducible on: 2021.1.0a9, 2021.3.8f1, 2022.1.13f1, 2022.2.0b4, 2023.1.0a5
Not reproducible on: 2020.3.38f1, 2021.1.0a8

Reproducible on: macOS 12.5