Currently Active Roles does not support scheduling future temporal group membership, while a user is currently a member or temporal member of a given group. When this is attempted this results in the immediate expiry of the existing membership or temporal membership.
This is by design. Enhancement Request ID: 727816 has been created detailing the feature above.
The product team will evaluate the request and this feature may become available on a future release of the product.
Please refer to this article for updates or contact support referencing the Enhancement Request ID: 727816.