When configuring Azure AD as the Single Sign On option (Setup -> Admin -> Single Sign On -> Azure AD) ( https://docs.vmware.com/en/VMware-Aria-Cost/SaaS/using-and-managing-vmware-aria-cost/GUID-managing-sso.html#enable-azure-active-directory-sso-25) for your Customers tenant please ensure you do so through a User account using the customers domain.

The Setup -> Admin -> Azure AD option when selected will claim the domain of the user currently activating SSO, so if done so from a Partner User the Partner's domain will be claimed and block sign in to the Partner tenant.

E.g. 

Partner User 1@partner.com

Customer User 2@customer.com 

If 1@partner.com switched into the Customer tenant and ran through the SSO process, the "partner.com" domain would be claimed under the Customer tenant. If 2@customer.com signed in and ran through the SSO process "customer.com" would be claimed. 

Note: This only applies for the Setup -> Admin -> Single Sign On -> Azure AD option, the SAML option will allow the domain to be claimed to be specified. Azure AD SSO can be configured via the SAML option by following - https://support.cloudhealthtech.com/hc/en-us/articles/360053181471-Is-there-a-step-by-step-process-for-SSO-Azure-AD-SAML-Registration-