I can confirm that the bug with shriekers will occur with any mod, even a blank one. Here are the minimum steps to reproduce it:
1. Launch the Divinity Engine
2. Create a new mod, type: add-on target: story
3. Skip creating an area
4. Open story editor
5. Compile the story
6. Save and exit
7. Launch the game
8. In the Mods, menu, select only the mod you created
9. Play through the game until you reach the Abandoned Camp waypoint
Without other mods, this will take some time. Here's probably the quickest and most reliable way to accomplish it without any other mods. (This should be in spoiler tags, but I'm not allowed to use them yet since I just registered):
- Play in Explorer mode, as there will be two mandatory fights
- Create an Inquisitor character with Lone Wolf just in case
- Rush through the ship, pick up a weapon after the scripted event
- Go upstairs, fight, then go to the boat
- Pick conversation options not to go back to the others
- Go straight to Fort Joy, ignoring everything
- From the square, go west and get the teleporter quest from Gawin
- Pick up Beast
- Kill the indicated enemies to get the teleporter gloves
- Return to the square, go down the hatch, talk to the dwarf with Beast in your party to get a key for the side door on the second floor of the fort
- Return back up, then go east behind Nebora's tent
- Teleport to the ledge, drop the ladder and head inside
- Enter sneak mode before opening the inner door
- Sneak all the way around the hall to the tower opposite of where you started
- Move the crates to get out, then drop down the ladder
- Go east along the coast, south of the fire, through the area with the talking skulls
- When you reach the edge, turn north
- Go north through the area with the pigs up to the broken bridge (avoid getting 'sploded by traps)
- Climb up to the bridge
- Walk straight east along the bridge, then keep to the southern path to avoid Loke
- Finally turn north to get to the Abandoned Camp waypoint
If you do all this in one go, the shriekers to the north will be visible, but crouching on the ground instead of hanging on the poles.
If you save and reload now, the shriekers will become invisible.
Note: The shrieker to the west of the Ruined Castle is not affected by this bug.
I can provide a savegame at the Abandoned Camp waypoint and the blank mod I created for testing purposes. Although testing will go quite a bit faster if you create a mod that lets you teleport to the Abandoned Camp waypoint directly. :)
