There should not be any problem with first taking withdrawals for expenses not yet paid, and I have never heard of a problem doing this, as long as the qualified expenses are paid later in the same year that the 529 distribution was taken. It only makes sense, right? If you need money from the 529 to pay expenses, you have to get the money from the 529 first.
Go ahead and do what you want to do, but please come back here in two years after you take the late 529 distribution and let us know if the IRS sent you a CP2000 notice and what the final outcome was. The main issue is probably that the 1098T from the school showing expenses paid and the 1099Q from the 529 administrator showing distributions won't match up in the same tax year, and this will get the attention of the IRS. You may be able to mollify the IRS with invoices, receipts, etc., but why jump through all those hoops when the whole mess can be easily avoided?