The “Release” button disappears when either a buyer or seller escalates the dispute to the Agent. Once you click the “Invite an Agent” button you give the Agent full control over your case and thus, no longer hold the right to release the amount from the contract.
Because this is not a bug, there is no way to fix it.
That said, the Agent decides based on the evidence provided. So, you may submit URL links to screenshots or other proof to support your claims in the chat associated with the disputed contract.