Can't complete a clean install of macOS High Sierra from USB. Macintosh HD not becoming a start up disk

I am trying to do a clean install of macOS High Sierra. I've done clean installs of macOS plenty of times, but I have never had this problem. I am booting into my installation media on USB. The installation tools appear as they should. I'm using Disk Utility to erase Macintosh HD into a clean APFS drive. I then select Install macOS and follow the prompts, select Macintosh HD, and it restarts. However, Macintosh HD does not become a startup disk -- I just reboot into the USB installer media. In Disk Utility, there's 5GB of content on the disk, but it does not appear in the Startup Disk menu nor if I hold option on boot. What's going on here? First Aid says the drive is fine.