When a Service Contract is setup, it will only post to the sales GL mapped in the Service Contract Account Group Code. The issue with posting to the single GL associated in the Service Contract Account Group is that when there is a Service Contract with multiple Service Items, each Service Item can be mapped to Items that have different General Product Posting Groups (which we know drives the P&L postings in BC). There is no granularity of the Items sold and consumed when everything is linked to the one sales GL used in the Service Contract Account Group.
Instead, the Service Contract postings should be driven based on the Item's General Product Posting Group so that it has consistency with the way Business Central drives postings for Sales and Purchase documents of Items.