Custom kong lua plugin loaded via helm config map fails with declarative_config.lua:654: table index is nil

Question

Kong Version 3.5

I'm currently developing a custom lua plugin that needs to be deployed via helm & a config map. We are running kong in dbless mode.

I created a config map

apiVersion: v1
kind: ConfigMap
metadata:
  name: kong-plugin-test-plugin
  labels:
     {{- include "test.labels" . | nindent 4 }}
data:
  handler.lua: |-
{{ .Files.Get "plugins/test-plugin/handler.lua" | indent 4}}
  schema.lua: |-
{{ .Files.Get "plugins/test-plugin/schema.lua" | indent 4}}

The file contents are

handler.lua

local TestPlugin = {}
TestPlugin.PRIORITY = 10
TestPlugin.VERSION = "1.0.0"
function TestPlugin:header_filter(conf)
  kong.response.set_header("TEST-HEADER", conf.header_value)
end
return TestPlugin

schema.lua

return {
  name = "test-plugin",
  fields = {
    {
      config = {
        type = "record",
        fields = {
          { header_value = { type = "string", default = "roar", }, },
        },
      },
    },
  },
}

I included it it my values.yaml

  plugins:
    configMaps:
      - name: kong-plugin-test-plugin
        pluginName: test-plugin

and its also mapped with these values to certain routes in my kong.config (kong.yml)

- name: test-plugin
  config:
    header_value: 'THIS-IS-A-TEST'

I checked the helm renderings and everything looks correct to me. However if I deploy it via helm, the container goes into an error state after logging the following

[error] 1#0: init_by_lua error: ...are/lua/5.1/kong/db/schema/others/declarative_config.lua:654: table index is nil      │
│ ack traceback:                                                                                                                             │
│   ...are/lua/5.1/kong/db/schema/others/declarative_config.lua:654: in function 'populate_ids_for_validation'                               │
│   ...are/lua/5.1/kong/db/schema/others/declarative_config.lua:639: in function 'populate_ids_for_validation'                               │
│   ...are/lua/5.1/kong/db/schema/others/declarative_config.lua:759: in function 'flatten'                                                   │
│   /usr/local/share/lua/5.1/kong/db/declarative/init.lua:224: in function 'parse_file'                                                      │
│   /usr/local/share/lua/5.1/kong/init.lua:533: in function 'parse_declarative_config'                                                       │
│   /usr/local/share/lua/5.1/kong/init.lua:715: in function 'init'                                                                           │
│   init_by_lua(nginx-kong.conf:47):3: in main chunk                                                                                         │
│ inx: [error] init_by_lua error: ...are/lua/5.1/kong/db/schema/others/declarative_config.lua:654: table index is nil                        │
│ ack traceback:                                                                                                                             │
│   ...are/lua/5.1/kong/db/schema/others/declarative_config.lua:654: in function 'populate_ids_for_validation'                               │
│   ...are/lua/5.1/kong/db/schema/others/declarative_config.lua:639: in function 'populate_ids_for_validation'                               │
│   ...are/lua/5.1/kong/db/schema/others/declarative_config.lua:759: in function 'flatten'                                                   │
│   /usr/local/share/lua/5.1/kong/db/declarative/init.lua:224: in function 'parse_file'                                                      │
│   /usr/local/share/lua/5.1/kong/init.lua:533: in function 'parse_declarative_config'                                                       │
│   /usr/local/share/lua/5.1/kong/init.lua:715: in function 'init'                                                                           │
│   init_by_lua(nginx-kong.conf:47):3: in main chunk  

I have no clue if I'm doing something wrong or if kong is an issue and have found nothing about this error "declarative_config.lua:654: table index is nil" I tried the same with an existing open source kong plugin (by copying it - and renaming it) from this repo but had the exact same error. I would have expected that the plugin simply works as its really basic and should just jumpstart the development.