Hey Jonathan,
I've checked your site and found that in backend settings >> plan, in plan assignment you have selected the profile type to JomsocialMultiprofile and in plan assignment rule you created the rule as per Joomla user group which is wrong.
see attached screenshot.
Also in the plan assignment rule, you are assigning the same plan to the user, so it will create a double subscription for the same plan which is the wrong configuration.
This plan assignment is basically used to assign some plans automatically to users when they register at the site as per the Joomla user group/es profile type.
Refer to this document for more details:
https://stackideas.com/docs/payplans/administrators/plans/plan-assignment
What I understand is that you want to assign a Joomla usergroup to users on the basis of the plan they subscribe to. For this, you need to use the Joomla User Type app. You don't need to use Plan Assignment.
Refer to this document for more details:
https://stackideas.com/docs/payplans/administrators/apps/joomla-user-type
Update us if you still have any issues.