num_rows returning empty while result_array has results

Use $query->num_rows() (method), not $query->num_rows (property). In CI3, if the property happens to be empty, the method will set it and return the value.

Since the property is not documented, but the method is, it's unlikely you were supposed to use the property in the first place.

Bonfire

Practical CodeIgniter 3
CodeIgniter Testing Guide