Running AEM 6.2, and when I go into Search form Assets Admin Search Rail is missing. It was there several weeks back is gone. Also when I search for anything within assets the right rail search box shows nothing but a search text box. To my knowledge I could not delete the form via touch UI, so am guessing something in CRXDE?
I don't know if you have solved this issue yet; but I had the same issue and after comparing my prod and dev environments, I noticed this path was missing " /conf/global/settings/dam/search/facets/assets/jcr:content" in my dev environment for some reason. I created a package for that path and uploaded it to my dev instance and my issue was resolved.
Hopefully this helps you or others if they run into a similar problem.