Allow map/feature layers to have changes reverted or reset after choosing to "Save Layer" in Portal web map viewer.
Esri's documentation described this process as creating a new layer where "If you are saving the layer for the first time or saving a copy of the layer, you create a new item in the site with your layer configurations"
It would be helpful if the documentation was more explicit that Copy must come first, then Save if you do not want to overwrite the settings on your original layer.
Submitted an enhancement ticket with Esri Support:
Portal for ArcGIS 10.5.1
non-hosted feature layers