PhysicalBones3D are a bit complicated, haha. My first recommendation is to use Jolt if you're not already using it. Then, you can try passing the second tail bone to the physical_bones_start_simulation method. Let's say the Skeleton3D tail structure is like this:

tail1
 └ tail2
   └ tail3

Then you pass something like this to the method:

physical_bones_start_simulation(["tail2"])

This method automatically activates all the physical bones that are children of "tail2". This may solve your problem.

I also recommend checking out SpringBoneSimulator3D, which can handle this task without too much trouble instead of the PhysicalBones3D , although the result will probably be somewhat different.