US engineers can build a mortar trench but unable to build the mortar weapon itself.
Only when clicking on the empty trench with heavy shovel they are able to build it. Should'nt they be able to build the 81mm mortar without needing the heavy shovel?

Finland engineers and heavy engineers are not building the MG weapons at the concrete fort MG locations. Its just empty after they finish the building animations.

Fin Engineer [imgur.com]