Catchup 'happens'. One does NOT sign up for catchups. In your case khn, if someone who signed up for catchup terminates before deferring $18,000 in total, what do you do with the supposed catchup? unless you have a limit, if that person deferred $9,000 and elected an additional $3,000 for catch up, then the testing is for $12,000 and no catchup.
After all these years I find it surprising that payrolls are still separating catchup because that is a testing issue, not a payroll issue.