Hi,
This seems to be an expected behavior and this question has been recorded in o365 user voice here: Free/busy information not accesible from O365 tenant and Hybrid OnPrem/O365 tenant
And I also found some related threads discussed about the similar issue below:
Free/Busy O365 tenant with Hybrid O365 tennant -OrganizationRelationship
Free/Busy O365 tenant with Hybrid O365 tenant
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