I'm having the same problem as in this post, but with using Paypal and iDEAL: https://premium.wpmudev.org/forums/topic/new-members-not-being-added-by-m2
Members are not added to the membership list of Membership 2, adding them manually and assigning the membership type that has being paid for does not work, I can choose and select the membership type, but once I click the save button all changes disappear.
I tested it by paying for a subscription through Paypal, payment was okay > user not added to list. also, the payment option iDEAL doesn't work either. Seems like a communication error in both cases to me.
Can you take a look? I've already granted access to the website.