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 which displays "Summary of item being approved" followed by additional data from that item.

It does not display on the Service Portal because formatter objects are not supported.  Please visit our documentation for more information.

This is the expected behavior within the Service Portal design.  Please feel free to visit this existing enhancement request on our Idea Portal for voting and submitting your own business case.