Not so sure this will result in an application being returned.
A "no" response as to whether the applicant was required to file a return that year does not preclude the possibility that the applicant filed a tax return for that year. So, a "no" as to required to file for the same year as "yes" a return was filed is not problematic or otherwise does not make the application incomplete.
There are multiple reasons why even a resident might not be required to file a return for a given year. So "no" responses to this should, generally, not be a reason an application is deemed incomplete and returned.
There is no requirement mandating that applicants must have filed tax returns except that for at least four of the relevant tax years, the applicant must have complied with any requirement to file taxes. Thus, a "yes" response in the required to file field for three or more of the preceding six years, for which the filed field is "no" (or blank), would make the applicant NOT eligible on the face of the application, and that would either result in the application being returned or subsequently denied.
Leaving blank the field for noting whether or not a return was required might result in the application being returned as incomplete, which generally is the risk for leaving any item or field blank. Likewise if the required to file field is checked yes and the filed field is left blank.
Otherwise inconsistent or erroneous entries in these fields are more likely to result in questions later.
Also note, these responses may be shared with CRA, and CRA can verify or contradict whether returns were filed.