Demystifying Hybrid Free/Busy: Finding errors and troubleshooting

CountryPreacher 

You need to look into Hybrid Mesh scenario: https://techcommunity.microsoft.com/t5/exchange-team-blog/the-hybrid-mesh/ba-p/605910 

Cloud organization (let's call it EXO Org B) needs to have 2 organization relationships: one for your on-premises Exchange xxxcorp.com and one for your Cloud Exchange Online Organization:

New-OrganizationRelationship -Name Hybrid_CloudSide -TargetApplicationUri outlook.com -TargetAutodiscoverEpr https://autodiscover-s.outlook.com/autodiscover/autodiscover.svc/WSSecurity -FreeBusyAccessEnabled $true -FreeBusyAccessLevel AvailabilityOnly -DomainNames xxxcorp.mail.onmicrosoft.com,xxxcorp.onmicrosoft.com

I imagine Org B only has 1 Org Relationship for your custom domain (whose autodiscover points to exchange on-premise hybrid)

Get-FederationInformation -DomainName xxxcorp.com -BypassAdditionalDomainValidation| New-OrganizationRelationship -Name Hybrid_OnPremSide -FreeBusyAccessEnabled $true -FreeBusyAccessLevel AvailabilityOnly

And then Org B needs to have a mail user created for jdoe@xxxcorp.com (cloud target user primary smtp address) with external email address set to jdoe@xxxcorp.mail.onmicrosoft.com (that resolves to the organization relationship with xxxcorp.mail.onmicrosoft.com target domain). 

Or the end users in Org B would put directly in Scheduling Assistant jdoe@xxxcorp.mail.onmicrosoft.com and not jdoe@xxxcorp.com (which is pointing to your on-premises org). Org B can test this (put jdoe@xxxcorp.mail.onmicrosoft.com in Target Mailbox Email Address) in https://testconnectivity.microsoft.com/tests/FreeBusy/input