If your appended RT item were constructed by program, call rtitem.Update / rtitem.Compact before appending it.

If your original RT item to append exists at all in a document, call NotesDocument.ComputeWithForm(true, false) ... and verify the result of that run is "true" (no errors on form). If it isn't always true, call rtitem.Update / rtitem.Compact before trying to append anything to this original RT item. But don't do both to the original RT item.

Finally -- when you're done appending -- One more ComputeWithForm with the result. And if it fails, rtitem.Update / rtitem.Compact .

In short: Appending two RT items presupposes that they're both "pretty clean". These functions will try to make them "pretty clean" so nothing horrible happens. And yes, exactly this "disappearing paragraph" problem has occurred to me when appending RT items. I corrected it with a ComputeWithForm call after trial / error on all the above. So ComputeWithForm seems to do the best by me.

This code is called in Queryclose

Dim doc As NotesDocument
    Dim memo As NotesDocument
    Dim zalacznikiRTItem As NotesRichTextItem
    Dim KartaProjektu As NotesRichTextItem
    Dim RaportKoncowy As NotesItem
    
    Dim attDoc As NotesDocument
    Dim rtitem As NotesRichTextItem
    Dim memoItem As NotesRichTextItem
    Dim item As NotesItem
    Dim editors As Variant
    Dim s As New NotesSession
    Dim CRLF As String
    CRLF = Chr(10)
    
    Call source.Refresh
    Set doc = Source.Document
    Set db =doc.ParentDatabase
    If Not doc.IsNewNote Then
        Set  KartaProjektu = doc.GetFirstItem("Materialy")
        Call doc.ComputeWithForm(False,True)
        Call KartaProjektu.Update
        Set  RaportKoncowy = doc.GetFirstItem("Materialy")
        If Not (KartaProjektu Is Nothing) Then            
            Set attDoc = db.CreateDocument
            Call attDoc.ReplaceItemValue("Form", "MaterialyDodatkowe")
            Call attDoc.ReplaceItemValue("Author", s.UserName)
            Set rtitem = New NotesRichTextItem(attDoc, "Attachment")
            Call rtitem.Update
            Call rtitem.AppendRTItem( KartaProjektu)
            Call attDoc.MakeResponse(doc)
            Call attDoc.ReplaceItemValue("Editors", "[ZATW]")
            Call attDoc.ComputeWithForm(False, True)
            Call rtitem.Update
            attDoc.SaveOptions=0
            Call attDoc.Save(False, True)            
            Set item = doc.GetFirstItem("Materialy")
            If Not (item Is Nothing) Then
                Call item.Remove
            End If
            doc.MaterialyID=attDoc.UniversalID
            Call doc.Save(True, False)
            
        End If
    End If
    

... needing to launch the whole thing as an agent.

When you hold the doc open in foreground, even after it's been saved & is still in QueryClose, the Notes editor gets involved.

The easiest way to work around that issue is to launch an agent after the document has been closed.

Your QueryClose does not have the on-disk version of the richtext item in memory, so you're catching it in midstream. And it's gonna be unpleasant whatever you try to do within the QueryClose.

If you truly, really need to do something in the QueryClose that can't be extracted from the rest ... I would say try harder, but it's also possible you can squeeze something in by loading the on-disk document from an entirely different method, like GetDocumentByUNID(). It is very dicey what you end up with though.

... needing to launch the whole thing as an agent.

When you hold the doc open in foreground, even after it's been saved & is still in QueryClose, the Notes editor gets involved.

The easiest way to work around that issue is to launch an agent after the document has been closed.

Your QueryClose does not have the on-disk version of the richtext item in memory, so you're catching it in midstream. And it's gonna be unpleasant whatever you try to do within the QueryClose.

If you truly, really need to do something in the QueryClose that can't be extracted from the rest ... I would say try harder, but it's also possible you can squeeze something in by loading the on-disk document from an entirely different method, like GetDocumentByUNID(). It is very dicey what you end up with though.