As Adam mentioned, SPF policy discovery works differently than DMARC when it comes to subdomains: if no SPF record is found on a subdomain, no attempt is made to use the SPF record on the organizational domain; SPF will return none as the check result.
A subdomain typically represents a separate department within an organization, e.g., sales.company.com for the sales department, it.company.com for the IT department, support.company.com for customer support, etc.
As different departments have different functions, so do the services they use to deliver emails. For example, sales might use Outreach, and support might use Zendesk, etc.
Therefore, unlike DMARC, it's probably not a good idea to fall back to using the root domain's SPF record if no SPF record is found on a subdomain, as the subdomain should use very different services.
To rectify this, one should publish an SPF record on every subdomain that sends outbound emails.
Learn more here: https://dmarcly.com/blog/how-spf-works-with-subdomains