[Bullet] Concave collision shapes not working with areas

[mbrlabs]

Godot version:
3.2.3 and 3.3 rc9

OS/device including version:
Windows 10, Nvidia GeForce RTX 2060/PCIe/SSE2

Issue description:
Areas with concave collision shapes (generated in Godot via the editor) don't detect collisions. If you use the same shape for a StaticBody it works however. I also tested this with GodotPhysics and it works there, so it's a Bullet-only related issue.

Concave collsion shape (created with "Create Trimesh collision sibling" in the editor):

Convex collsion shape (created with "Create Single Convex collision sibling" in the editor):

Steps to reproduce:

• Create a bunch of cubes in Blender and export as obj
• Generate convex and concave collision shapes
• Add both shapes to an area and disable one while keeping the other enabled
• Concave shape detects bodies, convex shape does not

Minimal reproduction project:
Same MRP as in gifs: AreaConcaveCollisionBug.zip

[mbrlabs]

I just found #27970, but this issue is fixed (at least the MRP works there). Probably because it's such a simple shape. For more complicated shapes that are actually concave like in my example it's still broken.

[rburing]

The MRP in #27970 uses a box-shaped area to detect a trimesh (concave) static body; it is not the same as this issue, which is that a trimesh (concave) area doesn't detect anything (when using the Bullet physics server).