Since the proxy panel only allows create/delete (not modify), I attempted to delete the existing rule for tiles.wmflabs.org but I was met with a Error: Unable to delete proxy: tiles.wmflabs.org. Of note, the confirmation dialog didn't list the name of this entry either: You have selected: . Please confirm your selection. This action cannot be undone.
I experienced a similar issue with trying to adapt security groups on the maps project. @bd808 at that time was able to make the change.
I wonder if some of these entries are so old, that something has gone wrong with permission tracking somewhere along the way, causing my account to not have the right permission to execute the action or something.. not sure.