Trying to replace pane buff

Hi,
Im writing a plugin that first outputs into a vsplit and I want to replace that buffer with the string I receive from my job:

      newFname = fname.sub(fname, 1, -1* (string.len(ext) +1 )) .. ".diff." .. ext                                                                      
                                                                                                                                                        
      local buf = buffer.NewBuffer("Running query, please wait. Might take up to 1-2 minutes", newFname)                                                
      local newPb = buffer.NewVSplitFromBuffer(buf, bp)  -- added it to initlua                                                                                             
                                                                                                                                                        
      shell.JobSpawn(cmd, {pwd .. "assistant.py", fname}, nil, nil, onExit, newPb, newFname)                                                            
                                                                                                                                                        
  end                                                                                                                                                   
                                                                                                                                                        
  function onExit(output, args)                                                                                                                         
      local pb, fname = args[1], args[2]                                                                                                                
                                                                                                                                                        
      if output == nil then                                                                                                                             
          micro.InfoBar():Message(string.format("Running assistant failed (nil) %s ", output ))                                                         
          return                                                                                                                                        
      end                                                                                                                                               
                                                                                                                                                        
      local buf2 = buffer.NewBuffer(output, fname)                                                                                                      
      pb:OpenBuffer(buf2)                                                                                                                               
      micro.InfoBar():Message(string.format("Done" ))                                                                                                   
                                                                                                                                                        
      return                                                                                                                                            
  end                                                                                     

But it is not happening. No errors, but content remains the same. Tried it with the parent PaneBuff, content is replaced, but with the buf not buf2.

What am I doing wrong?
I'm looking for some guidance here.

Thanks!
Semirke

Reproduce the plugin flow using buffer.NewBuffer, buffer.NewVSplitFromBuffer, shell.JobSpawn, and pb:OpenBuffer(buf2). Start by tracing the pane and buffer objects passed through onExit, then compare the behavior with the parent PaneBuf case shown in the report. Done means identifying why the displayed content is unchanged and confirming the supported way to replace it.