What is SPF flattening?
Each include, a, mx, ptr, and exists mechanism in your record costs a DNS lookup at evaluation time. Flattening resolves those mechanisms ahead of time and replaces them with the concrete ip4/ip6 ranges they expand to. The result authorizes the same servers but costs far fewer (often zero) DNS lookups.
Solid SPF never folds dynamic mechanisms (exists, ptr) or non‑pass‑qualified mechanisms into static IPs, because doing so would change what your policy actually means. Those are preserved verbatim.