Account id is lost during paid membership subscription
Here are the steps to reproduce the paid membership registration problem.
1. Create usergroup VIP
2. Create sponsored plan for member that would change Registered group to VIP
3. Register via frontend
4. Second step, where you are asked to make payment. Ignore it and try to login.
You can login as regular member, that's ok. New transaction record is also added, but its account_id is empty, though it must have registered account id instead.
Please correct this.