Hello, the problem you hasd was because PureOS modifies initrd at first boot, you need to re-sign files in /boot the first time you reboot.
https://docs.puri.sm/PureBoot/GettingStarted.html#first-reboot
One way to solve this is to do a OEM factory reset of Pureboot;
https://docs.puri.sm/PureBoot/GettingStarted.html#oem-factory-reset