Missed Deferral Opportunity- Safe Harbor Plan

Client has a plan with a standard safe harbor match. Participant went on the system and changed her deferral percentage. She also increased her catch-up amount by $10. The recordkeeping system does not recognize both a deferral percentage and a catch-up so only the catch-up portion was picked up by payroll. Hence, since July 2021, only the catch-up amount has been taken out of her check. She just noticed this a year and a half later.

Since this is a safe harbor plan, what would the missed deferral calc be based on? In other words, would it be based on the 8% that she wanted or just on the maximum match of 4%?

It's based on what she elected. See Rev Proc. 2021-30 Appendix A .05(5) and (9).

That was always my understanding then I read somewhere that if it was a SH plan, the calcs were done differently.

I think that's in reference to a missed participant, not a missed change by an existing participant.

Out of curiosity, how was she able to enter an election specific to catch up contributions if the recordkeeping system doesn't track catch up and deferrals separately?