Currently only group manager can see requested time off on the on-call workbench.However as per documentation - https://docs.servicenow.com/bundle/newyork-it-service-management/page/administer/on-call-scheduling/task/approve-PTO-create-coverage.html "As a shift manager, you can review and approve time-off requests from your shift members." but its not happening.
The default OOB behaviour OOB for time-off approval is to send it to Group Managers. This is so as PTO approval was believed to be real group manager controlled rather than a person who can manage On-Call shifts otherwise.
In the mean time if you wish you can customize function
"OCRosterSpanApprovalUtil" script include to return list of other managers so that you can include shift manager as well.
Enhancement request raised to address this for future releases. -