I have two modules: homepage and offer.
On homepage I'm including component that resides in offer folder.
include_component('offer', 'recentProducts');
I have enabled caching for this component in offer/config/cache.yml:
_recentProducts:
activate: on
And nothing is cached. Also cache directory for this component was not created.
BUT... when I move this component to homepage module, and activate it in homepage/config/cache.yml then cache is working as expected.
So it seems that other module's components cannot be cached, only when component is called from same module it belongs to.
I have not checked but it seems that it is also "partial cache" issue.