Probably nothing official by Microsoft but a good direct routing partner should make the boundaries clear within what they offer to you as a service. e.g. is their role to just send voice traffic to your O365 tenant? Once they can prove it's successfully going into Microsoft, then it comes under the Microsoft support.
As with any 3rd party support, a good relationship is key so they should be helping you in case of any issues with Direct Routing and not leaving the customer to struggle. Good Direct Routing partners should have the experience and knowledge to help even if it might not be strictly in their remit.
Unfortunately, there's no accreditation needed for any IT partner to setup a direct routing platform even if they know nothing about Voice\Teams\SIP Trunks which is causing issues for many companies.