all.yml

# host_vars/all.yml
foo_master = "{{ groups['blah'][0] }}"

# ^ For some reason foo_master is ignored here

maind.yml

# roles/foo/defaults/main.yml
foo_master = "{{ groups['app'][0] }}"

maint.yml

# roles/foo/defaults/tasks.yml

- debug: var=foo_master

# ^ This value is always the default value from the role.