Unfortunately there are many issues related to cell groups structure: SE / Cell @ CellGroupData in Cells family

And my rules of thumb, unfortunately, are:

• only use CellGroup+TextCell+ExpressionCell and friends, which are a higher level collegues of CellGroupData and Cell, for small snippets rather than serious work. Why? I don't know, maybe I just can't handle them but writing your code in a low level form works:

  NotebookWrite[EvaluationNotebook[],  Cell@CellGroupData[{Cell["hello"], Cell["goodbye"]}]]
  

• don't expect much from Manual CellGrouping with respect to other related options. Same reason.

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