I did a search on the Geonet forums and it seems like this is a very common issue appearing in a lot of threads. One of my map is having issues syncing data back into ArcGIS Online. The data is fully hosted on AGOL and it kept asking for credentials. Some of the field workers have had this issue before which I was able to fix but I'm stuck this time as it simply refuses to sync now.
Here are the steps I did to sort out the issue :
1. Used good Wifi connection to sync
2. Switched to cellular to sync
3. Logged out and in of Collector
4. Restarted the whole iPad
5. Entered the username and password
6. Resetted the password.
7. Enable editing for all layers.
8. Enable Push only in collector
From past experience, the steps above will solve the issue but it looks like it's a completely different issue this time.
As my data is on AGOL, I won't have access to server logs to check what is going on. On top of that, I was able to verity that the replica does exist in REST for the feature service.