I have made a map (new map viewer) and suddenly all the layers are in reversed order. Reordering the layers and saving the map did not work.
This behavior has a relation with this issue.
I have identified the reason why this issue occurs.
I also replied about this in https://community.esri.com/t5/arcgis-online-questions/cant-save-online-feature-layer/td-p/1125266 in this post:
In Map Viewer with an existing map it gives the error:
Doing the steps above in a new map, I can keep saving this map.