the_flexible_field() and has_sub_field() are deprecated. You should be using have_rows() https://www.advancedcustomfields.com/resources/have_rows/ for all repeater type fields, including flex fields.
For all sub fields of repeaters you should be using get_sub_field() https://www.advancedcustomfields.com/resources/get_sub_field/ or the_sub_field() https://www.advancedcustomfields.com/resources/the_sub_field/ and not get_field() or the_field()
have_rows() works on both top level fields and sub fields
