addendum, I briefly spoke with the dev:
In SetDParameter, there is code that sets the correct group automatically if force_type or force_type_mode is specified, and the other way around. One thing to keep in mind is that, when you set the parameter force_type_group, the mode is set to default value for that group, i.e., ball and socket for spherical, hinge for angular, slider for linear. This is what you see ig you play in with the dropdown menus in the UI as well. So, if one first sets force_type_mode and then force_type_group, the latter resets the mode to a possibly different value. But in the code snippet attached it looks though like the order is correct (assuming is the actual code used in the plugin).
In a way, I understand that if you are setting mode and group manually from you plugin and you know what you are doing, you are setting a combination that makes sense. But in general, to be extra safe, I decided to go with this solution, so if you change the group the mode is automatically set to something that makes sense. And, btw, also when you set the mode there is a check on the group, and if the values don't match it gets overwritten with the correct group.
so, valid options are:
simply set force_type (as before)
set force_type_group and force_type_mode (in this order)
only set force_type_mode (group set automatically)
If any of these don't give the expected combination it is a bug
I think he too struggled a bit with understanding what you did concretely, so I would recommend creating a bug report as lined out above.
Cheers,
Ferdinand