A user has approval records in the table 'sysapproval_approver'. However, he/she cannot see the OOB Approvals menu item in the Service Portal header. But the user can see the list of approval records from the list.
Release or Environment
There might be a Before Query business rule on the 'Approval for' record(example: RITM, change_request) of sysapproval_approver which denies access to the user. Since the user cannot see the record that they are approving, the approval records will not show up in SP header menu. Hence the menu will be missed.
Modify the business rule in order to provide access to the 'Approval for' records.