question

FlexSim's bone animator limits the number of bones-per-mesh to 42.

The disfiguration you are seeing is because your shape has a mesh that is being affected by more than 42 bones.

You can fix this in 3ds Max by splitting the meshes in the shape into separate meshes. For example, if the hands, head, and feet are all part of a "body" mesh, you can split the body mesh into separate meshes: hands & head/feet.

Here are a set of instructions that I wrote a while ago when I fixed this issue with another mixamo shape:

You'll want to save your shape before you start this process so that you can easily go back to a pristine condition if stuff messes up.

1. Make sure you are at frame 0 and that the model is in T-pose at that point.
2. Select just the Body component.
3. Execute main menu option Edit > Clone. Select Copy and press OK.
4. In the hierarchy view, click the light bulb next to each component except the Body copy (probably named Body001) so that only the new mesh is visible.
5. Click on the Body001 shape to select it.
6. Click on the Editable Poly in the modifier list (Press Yes to the dialog that appears).
7. Click the flat red box to enter face selection mode.
8. Drag a selection around the head and legs.
9. Press the Delete key to delete those faces.
10. Move the frame slider at the bottom to make sure everything is still animating correctly. If anything is messed up, reload the file and try again.
11. Repeat steps 5 - 10 for the original Body shape, but this time delete the hands instead of the head and legs. That way, the head and legs will be on a different mesh than the hands.
12. Export the mesh as an fbx file and test to make sure it works in FlexSim.