How to reproduce:
1. Create two Directional Lights in the scene (or any other object with Light Component)
2. In Inspector, set the Culling Mask of the first Light to 'Everything'
3. In Inspector, set Culling Mask of the second Light to 'Default'
4. Multi-select first and second Light

Expected result: Both Lights Culling Mask properties stay the same
Actual result: First Light Culling Mask property changes from 'Everything' to 'Default'

Reproduces with: 2018.2.0a6, 2018.2.1f1, 2018.3.0a5
Does not reproduce in: 2018.2.0a5

Notes: Selecting more Lights results in all their Culling Mask properties change corresponding to the last selected Light

Update: Fixed in 2018.3.0a9 and backported to 2018.2.7f1