In Arc Pro 3.0, the Flip Lines geoprocessing tool appears to be stuck in a "running" state, and canceling the tool gets stuck canceling. I am going on 14 minutes now, so the wait is real. I have attempted this same operation twice, and each time, I needed to end Arc Pro by the Task Manager.
This tool is working on 4 selected lines in a Road Centerline feature class, which is contained in an enterprise geodatabase in MS SQL Server.
I have not had the occasion to try flipping individual lines by editing vertices yet. I might report on that later.