Yes, there’s a special rule that you apparently are not aware of. FAFSA considers student-owned 529 accounts (includes custodial 529 accounts opened with UGMA/UTMA funds) to be parent assets, and they are to be reported as such in the parent financial section of FAFSA.
See the “Tip” at the bottom of page 72 of Completing FAFSA 2015-2016:
https://studentaid.ed.gov/sa/sites/default/files/2015-16-completing-fafsa.pdf
Schools that use CSS Profile can treat student-owned 529 accounts any way they wish, for the purpose of distributing institutional aid.