When I create an ad hoc metric, I can see it in purple in the far left panel and in the panel next to that where I have pulled in dimensions and measures to my visualization.
From either panel, I can access the drop down menu that provides an “Edit metric” option. For the past few days, if I edit the metric from the far left panel everything works as expected. If I edit the metric from the visualization panel my changes do not stick.
This is not blocking because I can make my edits from the left panel, but thought you should know about it.