Solution is:

• one must expose tags to be able to be used in groups using 'compose'

• groups cannot be nested under compose

Example:

compose:
  tags: tags

groups:
  tag_Role_monitoring: >
    'prometheus' in (tags.get('ansible-tag-role', '') | split(',') | map('trim')) ...

I guess Ill leave off with a rant/comment to the Ansible developer community: why not name it 'expose' instead of 'compose' but more importantly why make tags available to keyed_groups, but not groups by default?!