I'm trying to use updateDefinition on a hosted Feature Layer in order to re-enable sync, however nothing seems to be working. My postman, along with results, is:
I've used similar post requests in order to upload, download and publish files, however this seems to knock the sync off.
Are you able to re-enable sync on the layer through the GUI or from the admin endpoint in a web browser?
If this issue is still occurring, I would recommend reaching out to Esri Support. Let me know and I can assist in this process.