Fossa Union collision not working

Hello forum,

  1. What do you want to achieve?
    I want to make a fossa which would be unlocked by a button press.
    To make this fossa I will need to union BasePart with “FossaParts”

    (Those parts form a square around a castle) Here you can see the green baseplate and the red “FossaParts” which are already negativeParts (Separated). Now when the button is pressed I run this code :
local union = self.Baseplate:SubtractAsync(self.Instance:GetChildren())
for _, fossPart in ipairs(self.Instance:GetChildren()) do
union.Parent = workspace
union.Name = "Base"

Now this gives no errors but,

Vissually it looks just as I wanted, but the problem is in the collisions.

How can I make this so the player can fall inside of the fossa?
If you have any questions, please ask

You should change the CollisionFidelity to PreciseConvexComposition in the properties of the negate part or try other options for CollisionFidelity. Here you have the documentation of it CollisionFidelity.

I have this already set, but it doesn’t work

Then make the pit in studio and make it when you do the action you set the parts above the pit transparent

I use this kind of tycoon spawn system + the fossa triggers on button, so I would really want it to work this way (unioning it in the game)