Our extension makes use of some symfony bundles, those bundles include some routing.yml files which are used for testing and other purposes that should not be automatically found and loaded.
Currently the \phpbb\extension\finder is too general by scanning for all config/routing.yml files, finding all kinds of files that have no reason to be included.
The easiest solution would be to limit the depth of the finder. /ext/vendor/extension/config/ + 1 should be sufficient.
Another possible solution would be to exclude the vendor directory completely. Any routing files in those paths would need to be included from within the main routing.yml file.