We hear that a lot…and it’s rarely true. On extremely rare occasions a 503 comes from Cloudflare, but you would see a 503 Cloudflare Error in the response body. Your 503 had no body text.
I suggest you ask your host again to look harder (they generally won’t). You can also open a Cloudflare ticket to see if they can replicate the issue:
To contact Cloudflare Customer Support, login & go to https://dash.cloudflare.com/?account=support and select get more help. If you receive an automatic response that does not help you, please reply and indicate you need more help.