Net amount not calculating for event registration
Welcome!  Thanks for reporting that.  As you noted, in 5.6 the net amount is not displayed on those forms now since it was being automatically calculated anyway.  I have recreated your issue and it seems to be related to contributions generally rather than specifically related to events.  The net amount is being calculated correctly when the contribution is first created but edits to the fee amount are not causing re-calculation of the net amount.  I have created an issue here: https://lab.civicrm.org/dev/core/issues/552