Steps to reproduce:

1. Open attached project "GIParameters.zip"
2. Open scene "repro"
3. Open lighting window
4. In hierarchy, select "Cube"
5. In Lighting window, open "Object" tab and notice that "Advanced Parameters" are set to "Default - HighResolution". This means that it won't depend on "Default Parameters" value.
6. Now select "Plane" and notice that its "Advanced Parameters" is set to "Default - Medium". Its name implies that it also shouldn't depend on "Default Parameters" option
7. Open "Scene" tab of "Lighting" window
8. Notice "Default Parameters" is set to "Default - HighResolution"
9. Build lightmap
10. Set "Default Parameters" to "Default - VeryLowResolution"
11. Rebuild lightmap and notice that this time it's different. This means that some object in the scene uses value of "Default Parameters"
12. Select "Plane" and change its "Advanced Parameters" to "Default - HighResolution" and rebuild lightmap
13. Notice that scene looks like when "Default Parameters" had value set to "Default - HighResolution" => This means that "Default - Medium" isn't independent Lightmap Parameters but actually a value of "Default Parameters" field

Long story short - Change from "Default" to "Default - Medium" made it look as if "Default - Medium" is new Lightmap Parameters setting, where in reality it still points to "Default Parameters" field.

Regression introduced in: 5.4.0b2
Reproduced with: 5.4.0b3