How to reproduce:
1. Open the attached "1305595_Repro" project
2. In the Project window select the "New Material" Asset
3. In the Inspector at the top-right press the vertical ellipsis and select "Debug"
4. In the Inspector change the Lightmap Flags value to 3
5. In the Inspector at the top-right press the vertical ellipsis and select "Normal"
6. Repeat step 3

Expected results: Lightmap Flags value is 3
Actual results: Lightmap Flags value is 7

Reproducible with: 2018.4.30f1, 2019.4.18f1, 2020.1.17f1, 2020.2.2f1, 2021.1.0b2, 2021.2.0a1

Notes:
- The issue is not reproducible when the Material's Shader is any other than Standard and Particles Shaders (on which the Lightmap Flags value become 0 instead)
- Here are the other value changes: 2 > 6, 3 > 7, 5 > 1, 8 > 0, 9 > 1, 10 > 14, 11 > 15