Tim069 I have written to the support, because with the account model Joint only the first "main account" is supported, but not the joint accounts and savings targets.
The answer took a while, but came now:
Dear sir […],
bunq accounts are accessed by Banking4 through the API.
At the moment, however, only "normal" accounts of Banking4
supported. We have extended Banking4 to the future
also savings targets and joint accounts with Banking4 set up online
and can be retrieved. Stand with the next update to Banking4
these extensions are available.