Not stopping Match/SH when comp goes over limit

Is there specific guidance anywhere that says Employers should not stop payroll matches when the employee reaches the max compensation during the year?

For example, plan matches dollar for dollar.  I don't see why the Employer should stop (testing reasons notwithstanding).

Or, maybe someone started deferring in March with a 50% of deferrals no more than 6% of comp (for a cap of $10,350 on $345,000 in comp).  They hit $345k in September, but hasn't hit that match cap yet.

We know they are supoosed to keep going until they hit a cap, but is there anywhere that says it outright?

Unless the plan documents says otherwise, this paragraph from the preamble to the 415 regs is what I have relied on when this question comes up. You are not required to count compensation on a FIFO type accounting basis. 

https://www.federalregister.gov/d/E7-5750/p-111

"As noted above, the final regulations provide that a plan cannot take into account compensation in excess of the section 401(a)(17) limit. In addition, the final regulations provide that elective deferrals can only be made from compensation as defined in section 415(c)(3). However, in applying these two rules, a plan is not required to determine a participant's compensation on the basis of the earliest payments of compensation during a year."

Not opining, but I can tell you that Datair does exactly that.  If you are processing payroll in Datair and calculating the match every pay period, it will not calculate the match if a participant's compensation has gone over the limit.  Fortunately, I only have one plan like that and it has a true-up provision in it.  So all is well in the end.

You might also find this helpful. https://www.irs.gov/retirement-plans/401k-plans-deferrals-and-matching-when-compensation-exceeds-the-annual-limit