How to reproduce:
1. Open the attached "Unity2018_3_PrefabsIssue.zip" project
2. Go to Project tab and open 'GameObject' prefab in Prefab mode
3. Change any of the parameters (for example Activation Delay) of 'Coord Animation (Script)' component
4. Click on another asset and when open

Reproducible: 2019.1.0f1, 2019.2.0a11, 2018.3.14f1 (b2f2b088a971)
Not reproducible: 2017.4.25f1, 2018.3.0a5, 2019.3.0a2 (fa7740529556), 2019.2.0a15 (aba135cd5df5), 2019.1.1f1 (ab42174bc696)

Workaround: Disabling/enabling Atavism component saves prefab

Notes:
- Atavism asset: https://assetstore.unity.com/packages/tools/network/atavism-2018-3-op-standard-117342