If their DMARC alignment relies on SPF only, it will break. But if it relies on DKIM (far more common) or both SPF and DKIM (best practice), forwarding won't cause any issues.

If your email breaks when forwarded, your setup is broken. Tons of people use Cloudflare Email Routing or similar services; you must account for them.

That being said, I forward mail addressed @mydomain.com to my Gmail, and I've had a couple of cases where legit messages landed in spam because it was SPF-aligned only.