Steps to reproduce:
1. Create or open any Unity project
2. Create/Select an object with a Camera component
3. Open "Culling Mask" dropdown and disable all the flags
3. Observe the component's "Culling Mask" property value in Inspector

Expected: Property value is shown as "Nothing", opened dropdown has a checkmark next to "Nothing"
Actual: Property value is shown as "Mixed...", opened dropdown does not have a checkmark next to "Nothing"

Reproduced with: 2021.2.0a3, 2021.1.0a5
Didn't reproduce with: 2021.1.0a4, 2020.2.0f1, 2020.1.17f1, 2019.4.17f1, 2018.4.30f1

Note: reproducible with in other editor places as well ("Light" component, "Platform Effector 2D" component, Window->Analysis->Physics Debug window, etc)