We've had the same postgresql (15.6) database and feature service published for a few years, its been working great and is heavily utilized by staff using the Field Maps app (both iOS and Android).
After upgrading to 11.3 we're getting failed sync's and consistent error logs in server manager.
Field-collected edits are successfully synced to the SDE geodatabase, but the users are unable to sync in the download direction (from SDE geodatabase to Field maps device). A lot of edits/manipulation is done when the upload sync happens via attribute rules, and the field staff are UNABLE to then sync these edits to their devices, causing field-chaos.
I have a case open with tech. support, just seeing if any other users are experiencing this behavior at 11.3
We have been having a similiar situation since upgrading our stand alone ArcGIS server to11.3. Our existing editable, synced RESTS which are referencing layers in SDE stopped syncing in field maps. We have attempted recreating them in several way with ESRI assistance but no luck.
Were you able to resolve your issue with ESRI's assistance. We have a support case open but the tech cannot figure it out.