[Full disclosure: I'm an employee of Cisco and my teams works on the OpenDNS resolvers. View expressed here are my own though informed by my history and exposure to the product.]
Sounds like you may be hitting a ratelimit. We only use REFUSED in a few cases, primarily when rate limits have been violated but also if the domain in questions appears to be part of certain attack patterns (I don't think that's the case here.) Do you have an OpenDNS account or are you just pointing to OpenDNS without an account? Accounts have higher ratelimits (as we have a means to contact you if things are going sideways.)
support@opendns.com is always an option to, but I doubt it's anything specific to Pi-Hole that's causing it.