No field in which to add Retina logo in Site identity

I've been having issues with changing logos in Site identity on a staging site (using wp-staging on the main site). At first trying to change the logo would force a 404 page when publishing, and on reloading, would not show the change.
The last time, however, the field for the retina logo simply disappeared. See this screenshot: https://www.dropbox.com/s/26r3icxn0n81pvf/Screen%20Shot%202018-12-23%20at%2010.31.12%20AM.png?dl=0

I've tried turning off all plugins, no change. Turned GP Premium back on. No change. Changed themes, no change. Cleared Varnish and Browser cache. Looked at it in both Chrome and Firefox. I'm out of ideas...