write_files for curtin and write_files for cloud-init - using both seems to conflict

In my /etc/maas/preseeds/curtin_userdata_custom file, I have a write_files object that specifies a user with a public key using the object key ssh_authorized_keys key. This has traditionally worked with maas environments we have running. In fact, this is deployed using a config we haven’t changed. It seems now though that we’re deploying machines with a user_data package which now also uses write_files and now only one of the write_files works (the one in user_data).

Is this a known thing where write_files can only be used once during a deployment?