Technically that message does not tell me it's completed, but that a new one is available(this is where my confusion started).
So I loaded the save game again, re-did everything, looked in the "Stronghold - Status" and found out what's happening.
Apparently I get a 2nd adventure while the other one is running, but since I turned in lots of bounties the running one is resolved at the time making it look like it gets reset.
Thanks for your patience in solving my non-existent "bug". (somehow kind of glad that the one from this thread (probably) is one, so that I'm not wasting your time).