In a SSJS an LS-Agent (noteID as param) is called. This agent uses various LS-Functions to modify the passed document. The document is saved at the end of the agent
As far as i understood, the SSJS waits till the agent completes.

After calling the agent a value is read from the document.

var agent:NotesAgent = db.getAgent("agentName");
agent.run(nid);         // LS-Agent
readValueFromDoc(doc);    //SSJS Function in a ScriptLib -> read value X from document

The item is not available in the document! No matter if i recycle the doc and re-read it with database.getDocumentByUNID()....

Solution:
If i copy the document to the database via

var copyDoc:NotesDocument = doc.copyToDatabase(db);

the value can be read from the copyDoc.

The workaround does what it should, but I'm not happy with it....Any explanations or hints? Thanks