I have a similar situation.  I'm likely going to hang my hat on this Notice of Proposed Rulemaking (and disclose reliance on it):

https://www.govinfo.gov/content/pkg/FR-2008-01-18/pdf/E8-859.pdf

See PDF page 5, last paragraph, continuing onto page 6:

Section 529 is silent regarding
whether distributions must be made
from a section 529 account in the same
tax year as QHEEs were paid or
incurred. Concerns have been raised
that individuals could allow the account
to grow indefinitely on a tax-deferred
basis before requesting reimbursement
or use distributions in earlier years to
pay QHEEs in later years. Accordingly,
the IRS and the Treasury Department
propose to adopt a rule that, in order for
earnings to be excluded from income,
any distribution from a section 529
account during a calendar year must be
used to pay QHEEs during the same
calendar year or by March 31 of the
following year.