As far as explicit guidance from the CSS Profile itself, that’s harder to find. My guidance would be in the absence of explicit documentation stating otherwise, to declare assets consistent with federal definitions and rules that have been in place since 2009.
This mentions the origin of when this rule went into effect:
"Section 529 College Savings Plan asset of account owner (low impact if owned by student or parents; high impact if owned by a third party)…Custodial 529 college savings plans owned by a student, where the student is both the account owner and beneficiary, are reported as a parent asset if the child is a dependent student and a student asset if the student is an independent student. This treatment is effective with the 2009-10 award year due to a legislative change enacted in the College Cost Reduction and Access Act of 2007. "
https://finaid.org/savings/accountownership/
It also states that this is not the case for other types of custodial accounts: “UGMA/UTMA Custodial Account: asset of beneficiary (high impact)”
Could a CSS Profile school treat a dependent student-owned 529 as a student asset? Sure. But I haven’t seen any cases of this in my own experience talking to at least a half-dozen FA offices directly about this issue, or in multiple on-line forums where the universal experience is the CSS Profile schools treat them as parental assets. Unless someone presents evidence to the contrary, I wouldn’t think is is a likely occurrence. If there is a school that in inclined to do that, they would ask specific followup questions about the declared parental-owned assets anyway.
In filling out the CSS Profile initially, there would no reason not to properly declare a custodial 529 account of a dependent student as a parental asset.