Welcome back to the Wefugees Community and thank you for reaching out to us.
It is a bit difficult to answer your questions without knowing your situation/background better. However, since the immigration office explained already to issue a Duldung, it should be a good sign.
Authorities can send the invitation letters for the voluntary return counselling quite randomly and without a specific trigger. At times authorities even hand out information letters on that kind of services before the asylum interview ("Anhörung").
However, if you worry too much, I can recommend you to consult a migration counselling office for some legal advice and/or psycho-social support. This will be the best option to discuss your case individually and confidential.
Please also don't hesitate to get back to us with any further questions.
All the best,