Composer 10.26.1 Packages Won't Install with startosinstall --installpackage

I use Composer to create firstboot scripts for our Macs and recently I noticed they weren't installing with startosinstall with the installpackage flag. There was no telltale sign in install.log but I knew that the older packages still worked. Using the process of elimination, I deduced that the issue was with Composer 10.26.1 as I was able to get the packages to install if I made them with Composer 10.20.0. I don't know at what version between 10.20 and 10.26 where it broke, so YMMV.