Patient: Doctor, it hurts when I do that.
Doctor: Don't do that.
Seriously?
Either this is intentional (which I doubt) and the participants previously received notification that a deferral election can serve to sever payroll deductions for other benefits and that in such case they would have to make other arrangements to avoid interruption of those other benefits or it is unintentional, in which case the plan sponsor should look to the controlling plan documents to see if there isn't some way to justify them changing the order of withholding, which I can't believe they can't find.
If need be, amend the plan.
But I find it hard to believe that will be necessary. Here is some language I found in a volume submitter plan:
The rules and procedures for allowing Participants to defer and reduce their Compensation under
Paragraph xxxx of Article I and this Paragraph xxxx and to decrease (or increase) such reductions and deferrals shall be established by the Committee, in its sole discretion; provided, however, that the Committee shall exercise its discretion in a uniform and nondiscriminatory manner. "
If your plan has something similar it should allow the Committee to establish a rule that says: the maximum deferral for any pay period shall be no more than net pay.