TurboTax is unable to flag an excess elective deferral to a 457(b) because 457(b) plans allow special catch-up contributions that you may or may not have been eligible to make. If you don't qualify for any of the special catch-up contributions, yes, you also have an excess contribution to the 457(b) plan. 401(k) plans allow no similar special catch-up contributions, so TurboTax is able to detect when you have made an excess elective deferral to a 401(k) plan.
https://www.irs.gov/retirement-plans/plan-participant-employee/retirement-topics-457b-contribution-l...
TurboTax does not automatically add detected excess contributions to income, you must do that yourself as Other income.
