Okay, I have found the reason. The AI is set to a detection range of less than 1000m.
This is utterly R I D I C U L O U S and inexcusable! I mean the artillery in Il-2 was already far from optimal but this here is absolutely and completely nuts. The darn howitzer had a maximum range of 10,675 kilometers! Where is the sense in creating such beautiful ground objects and then giving them parameters that make em blind, deaf and incapable of sensing anything? Yet one more serious issue with the FMB that really needs to be fixed. Artillery is a long-range indirect fire weapon and not a friggin' AT-gun.
Suggestion to solve the issue:
Allow the creation of a "target box" which guns such as the 10,5cm lFH 18 can shell from a distance so their role is depicted correctly. The user places such a box, assigns it as target to the howitzers and they shell the area. Nuff said.