Boot Camp Control Panel macOS partition doesn't show up
Since High Sierra, the macOS partition is formatted APFS. Due to what I presume is an oversight by Apple, the Boot Camp Control Panel only looks for HFS+ partitions as suitable startup disks. Therefore macOS partition won’t show in Boot Camp Control Panel since High Sierra.

To set macOS as default, use the Startup Disk preference pane in macOS System Preferences, or hold control as you select the disk from Startup Manager (the alt boot menu).