Comment 3 for bug 1859049

Thanks for the bug Sean

As you said in your description:
"linux-signed-* packages were transitional and were removed in the 5.4 upload therefore curtin should not be trying to install them."

Your curtin config that you attached from maas for those nodes is specifically requesting that linux-signed-generic kernel package gets installed on that machine due to the curtin config it provides to the node:

kernel:
  mapping: {}
  package: linux-signed-generic

So, something in MAAS, or in /etc/maas/preseeds is sending that config to curtin which curtin fails on because that package doesn't exist.