How to reproduce:
1. Open the attached project "decalBug.zip"
2. Open "SampleScene" Scene and observe the Scene View
3. In the Project window, go to Assets, then right-click "DecalTex" and open the folder that this texture's in
4. Edit the texture on any drawing application, change its color and save the changes
5. Focus back to the Unity Editor and observe the Scene View

Expected result: Decal texture updates on transparent materials
Actual result: Decal texture doesn't update on transparent materials and requires entering and leaving Play Mode, or restarting the project to apply changes

Reproducible with: 2019.4.5f1 (7.3.1, 7.4.1), 2020.1.0f1 (8.2.0, 9.0.0-preview.33), 2020.2.0a19 (8.1.0, 9.0.0-preview.33)
Couldn't test with: 2018.4.26f1 (package errors)