Headless upgrade with `softwareupdate -iaR` says 'restarting' but doesn't restart

I'm trying to headlessly upgrade macOS from 14.7 to 15.0 on my M2 mini. The upgrade from 14.6.1 to 14.7 went smoothly. However, the same command doesn't work for 14.7 to 15.0 it seems. It says it's restarting, but it's not. A manual restart works, but the version is unchanged after the system is back up. How can I fix this? Here's the command I'm using:

$ sudo softwareupdate -iaR --agree-to-license
Software Update Tool

Finding available software
Downloading macOS Sequoia 15.0
Password:

Downloaded: macOS Sequoia 15.0
Restarting...
$

Note that _nothing_ happens after the Restarting... - it just returns the prompt - no restart happens.