To reproduce:

1. In a new project, create a new folder
2. In a new folder, create a prefab
3. Right-click on the prefab and select Create/Prefab Variant
4. Observe the name of the variant

Expected: the name should be "prefabName Variant" as it is when the original prefab is directly under the Assets folder
Actual: the name of the prefab variant becomes "Assets_Models_prefabName Variant"

Reproduced in 2018.3.0b1, 2018.3.0b2, 2019.1.0a1

Fixed in: 2019.1.0a6