Reproduction steps:
1. Open attached project "TexTileTest.CopyDXT.2019.3.zip"
2. Click on the Scripts/DXT/DXTTools.asmdef in the project so it shows in the Inspector
3. Make a change e.g.Tick the "Stadia" box to include that platform
4. Click on a different file in the project
5. Now you Will see the pop-up dialog, press Apply
6. Switch back to DXTools.asmdef

Expected result: Pressing Apply would apply the changes.
Actual result: Pressing Apply will set the Inspector to your new file, but not Apply the changes to the asmdef, the file is not changed, and when you click back to the asmdef the change(s) are lost.

Reproduces on: 2019.3.0b1, 2019.3.0a4, 2019.3.6f1, 2020.1.0a18
Regression introduced: 2019.3.0a4
Fixed in: 2020.1.0a19
Does not reproduce on: 2019.3.0a3, 2020.1.0a19, 2020.1.0b2, 2020.2.0a3