This has happened several times with several clients. Data is published to the AGOL account as a hosted feature layer. The layer is added to a web map. Sometimes the web map is used in a web app created with Web App Builder. Users in the AGOL account are required to make updates to existing line feature layers. This process works for about two months. Then, suddenly, the line feature that they have been updating and using for a while does not allow any updating of the vertices of existing features. You can add a new feature. You can modify attributes. Editing parameters have not been changed.
Esri tickets have been submitted in the past with no resolution. The only solution for a couple of the features was to completely republish the data or try to make copies using AGOL Assistant. This sometimes errors out and is not a solution.
This has happened with at least 5 separate accounts that I can think of on different computers and 3 different browsers.
Why does AGOL do this all of a sudden?