Are deletes allowed on the service itself? Some services can have specific permissions on them disallowing deletes.
What does List By Editing within the Contents pane show? Any warning badges?
What does the Editing Status window show? Any warnings, messages, etc?
More info:
https://pro.arcgis.com/en/pro-app/latest/help/editing/specify-which-layers-can-be-edited.htm
https://pro.arcgis.com/en/pro-app/latest/help/editing/view-layer-permissions-for-editing.htm
If no useful information shows, perhaps a call to Esri Technical Support can help you resolve it.
-Scott
You could overwrite the layer and make sure that you allow to edit the service without m-value