Actual behaviour:
The classical SEPA payment processor creates a (wrong) financial_transaction record in the table civicrm_financial_trxn, at the time the mandate is entered. These entries are marked as "completed", and have a wrong payment method.
When the SEPA group is marked as "received", another financial_transaction record is created, with the correct values.
This creates severe problems for use cases, that are using the table civicrm_financial_trxn: e.g. CiviCRM invoices, accounting export.
The issue is only happening with OOFF payments, not with FRST/RCUR payments.
Workaround:
With the SEPA NG payment processor, this does not happen.
However, the SEPA NG payment processor currently does not support event payments (see #17)