Payments, memberpayment plugin; Payment is done but user is not assigned in the right group

• Bug
• Regression
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I'm testing the PayPal capabilities of Tiki and this is about the "memberpayment" wiki plugin : https://doc.tiki.org/PluginMemberPayment

Following the doc I used the plugin as simple as possible as well as other requirements;
https://doc.tiki.org/Payment

Groups
I created a group;

• Enabled "User can assign himself or herself to the group"
• Set Membership expiry, Number of Days to 365
• Set Pro-rata Membership to "year".

• Payment system: Paypal
• Currency: Euro
• PayPal ID: paypal email
• PayPal environment: Production
• Enable PayPal instant payment notification (IPN)
• Enable Payment data transfer (PDT)
• Set the Payment data transfer (PDT) token
• Redirect after PDT payment: https://dev.domain.org/BernardPremium_homepage

{memberpayment group="BernardPremium" price="1" returnurl="https://dev.domain.org/BernardPremium_homepage"}

Use cases
Notes: payment are completed in all cases and it create an row in the Outstanding table (tiki-payment.php#content_payment1-1)

If a member of the Admins group subscribe himself and do a manual payment: He is assigned to the group. The user is redirected to tiki-payment.php?invoice=nn (almost good)

If a member of the Admins group subscribe another user and do a Paypal payment: The user is NOT assigned to the group, he is not returned to the Tiki site automatically. If he clicks on "return to merchant" the user is redirected to tiki-payment.php?invoice=nn (bad)

If a registered user and do PayPal payment: The user is NOT assigned to the group, he is not returned to the Tiki site automatically. If he clicks on "return to merchant" he is returned back to "tiki-payment.php?invoice=nn" (bad)

While I'm not getting into the code I understand 2 mechanism are invoked and not working.
Group assignment
Returning from Paypal

I'm not sure if they are required together to have the user assigned.

This bug has been demonstrated on show2.tiki.org

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This bug has been demonstrated on show.tikiwiki.org

Please demonstrate your bug on show.tikiwiki.org