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I still have this in 2019.4.8f1

Still present in 2019.3.15

still present in 2019.3.8f1 with custom forward renderer. Switching back to default renderer and reimporting worked for me, though I'm not sure for how long.

2019.3.13 and the bug is still there

Also there in 2019.3.9f1

Reproducible in 2019.3.4f1 as well.
Temporary workaround: change the default layermask to "Everything", right click in the project window near the prefabs, click "reimport", then change the default layermask back to what it was.

Pros: The previews will be back

Cons: Every time you edit a prefab its preview will disappear again.

Here's my temporary solution, I use Unity's default pipeline asset in editor and change it at runtime:

public class SetRenderPipelineAsset : MonoBehaviour
{
public RenderPipelineAsset RenderPipelineAsset;

void Start()
{
QualitySettings.renderPipeline = RenderPipelineAsset;
}
}

As a temporary hack I made a custom quality level pointing to a URP asset with a forward renderer w/o a mixed layer mask on it, and switch to it only while in the editor.