It seems to take a little "reading between the lines" in PUB 970, which has the following wording:
To determine if total distributions for the year are more or
less than the amount of qualified education expenses, you
must compare the total of all QTP distributions for the tax
year to the adjusted qualified education expenses.
Here's a good non IRS explanation: https://www.savingforcollege.com/article/timing-of-529-plan-distributions-must-match-qualified-expen...
