I'm not sure this is a bug. The upperdir is maintained by overlayfs as a delta between lowerdir and the mount target (i.e. a and c). You're not meant to play with the upperdir yorself. If you, after this experiment, unmount and remount the overlayfs, does the file then show up correctly?
I'm not sure this is a bug. The upperdir is maintained by overlayfs as a delta between lowerdir and the mount target (i.e. a and c). You're not meant to play with the upperdir yorself. If you, after this experiment, unmount and remount the overlayfs, does the file then show up correctly?