PayPal Express IPN Issue

I'm trying to get Membership set up, and can't seem to test in a way that activates a new member upon a successful transaction. There is a thread going (https://premium.wpmudev.org/forums/topic/new-account-not-active-after-successfully-registering-and-subscribing-paypal#post-170419) that gets to this issue. I've followed, and seem to have everything properly set up, but still no activation.

Here's what I have done:

Set up Membership with the appropriate PayPal Express account info
Enabled Test Mode (Sandbox)
Set up a PayPal developer Sandbox account, and successfully tested the IPN using the http://mydomain.org/paymentreturn/paypalexpress URL - got the green check
Ran through the Membership registration process, and successfully completed the transaction using a PayPal sandbox account.
Looked at the Members list in the Membership dashboard. The user account is created but not activated.

I really need to get a handle on this ASAP, and am hoping someone can quickly point me in the right direction.