Thanks very much for your knowledge Jeremy! I looked around at different x-affiliate posts and found what i was missing.
Hidden/Burried deep into the partner's user profile, there's a setting:
In the user's profile, there are 2 boxes under "affiliate plans"
Affiliate plans Affiliate plan [pulldown-menu w/ plan names]
Signup for partner plan [pulldown-menu w/ plan names]
What's interesting is that if you assign an affiliate plan in the AFFILIATE admin section, it only assigns the top "Affiliate plan" setting. There is no place for "Signup for partner plan" anywhere in the admin section.
When I assign "Signup for partner plan" to my default affiliate plan-- and then save, things seem to work.
These 2 fields are not available at signup -- even if the admin is creating the partner.
The "affiliate plan" assignment can be made from the x-affil admin, but "Signup for partner plan" can not.