Humanoid does not swim down at surface

I am currently trying to get my humanoid to be able to swim down. For what I am doing however, I should only have the Humanoid and HumanoidRootPart in a model. For some reason, when the model reaches the surface of water, it cannot swim down, but rather it just walks around at the surface

Here is an example of this happening: swimming issue

When adding a part that has a name is used in characters, this problem suddenly stops happening (Torso, UpperTorso, LowerTorso, Left Arm, RightUpperArm, ect.)

I would like to know why this happens, and also a workaround, if possible.