When creating a prefab from a gameobject with rigidbody rotation constraints, the constraint locks are not saved to the prefab and also reset on the object. Happens also when you update the prefab with the Apply button. The issue only occurs if there are 2 or less rotation constraints on the rigidbody.

Repro:

1. Open the RigidbodyConstraints project
2. Load 'testscene'
3. Select the 'Prefab' object in the hierarchy
4. Enable 1 or 2 rotation constraints on the rigidbody component
5. Click Apply to save changes to prefab
6. Observe rotation locks are reset

Happens in:
5.5.0b7

Working in :
5.4.1f1