(Posted to StackExchange a week ago, with no response.)
I have used the Option Inspector to set CellGrouping to Manual for the current, selected Notebook. Nonetheless, the following does not group these two cells together:
CellPrint[CellGroup[{TextCell["hello"], TextCell["goodbye"]}]]
Also, adding the second argument to CellGroup[] to specify which cell should be open, does not work.
Please note that both of these work if I create the cells in a new Notebook, created with CellGrouping -> Manual.
I saw the same behavior with Cell[CellGroupData[...]].
What am I missing?