Beginner question: Baked light on basic cube unexpected behavior.

I’m completely new in lighting and lightmaps, so trying to do basic things with lightmaps and in the meantime check how much I understand the topic. Here is the scenario:

I have a simple unity cube with Standard material applied with Albedo color green.
Mesh renderer has the following settings,


I have one directional light in that scene with mode Baked.
I turned off Real-time GI from light settings and using Progressive CPU lightmapper.
What i get after baking lightmap is following cube

as you can see it has unexpected black artifacts on the front side, while topside seems not to have it. Is this a normal behavior? When I turn on Real-time GI artifacts disappear and everything looks as expected.
How can I get the same result as with Real-time GI but without that feature turned on?

update:

forgot to mention that object is marked as static

It appears that in Window->Rendering->Lighting Settings there is a checkbox called Compress Lightmaps, so unchecking it solved the problem for me.