Connecting redmine issue https://projects.theforeman.org/issues/24145 from this bug
Moving this bug to POST for triage into Satellite 6 since the upstream issue http://projects.theforeman.org/issues/24145 has been resolved.
Verified in Satellite 6.4 Snap 12. Attempting to update the subscription now works as expected. Both the edit form, and the update task are successful.
Since the problem described in this bug report should be resolved in a recent advisory, it has been closed with a resolution of ERRATA. For information on the advisory, and where to find the updated files, follow the link below. If the solution does not work for you, open a new bug report. https://access.redhat.com/errata/RHSA-2018:2927