When you enter a Build Block on a ship, the first weapon slot on that ship lost its assignment.
All other weapon slots look to be unaffected.
To reproduce:
Build or take a ship and assign any weapon to the slot 1 in the main bar.
Now leave the ship core and enter a Build Block on that ship.
Leave the Build Block again and enter the ship core => weapon slot 1 is unassigned.
This also happens with two players, when one control the ship and an other enters a build block on that ship.
Now the pilot is unable to use the weapon slot 1.
Even if he reassign the slot, after one second it will be unassigned again.