Issue
The Service Portal displays [sysapproval_approver] records, but the form does not show the Approval Summarizer. On the native UI Form view, this is the formatter that displays "Summary of item being approved" followed by additional data from that item.
Cause
It does not display on the Service Portal because formatter objects are not supported. For more information, see this documentation.
Resolution
This is the expected behavior within the Service Portal design.
You can visit this existing enhancement request on the Idea Portal to voting and submit your own business case.
Related Links
View this community-driven workaround to get that data displayed on the SP approval: Custom Workaround Discussion