(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:
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: