The auto-complete function does not bridge empty cells. This allows a sheet to have several tables, of differing structure, not cross-pollinate auto-completions.
It's a feature.

Filling all the empty cells with a single quote mark (') will lend continuity. When adding rows, prefill all the cells with 's. (edit: A space will work as well)

Leading 's are formatting marks that have been around since the dawn of spreadsheets. (Try Visicalc) They are non printing and signify that the cell contents should be treated as a string.

A number entry preceded by a ' will be omitted from the sum() function. Excel typically flag's that with a warning.