Hi dkrishna,
From my test, it seems that Note(annotation) entity is not able to show as subgrid like other related entities, it is by design.
I have tried following approaches:
1. Inject a custom fetchXml to subgrid control, however, after having viewed many threads in forum, I found that this method was only supported in web legacy client.(getControl("xxx").setParameter("fetchXml", fetchxml))
I also tried answer of this thread, the author shared a solution for UCI, unfortunately it didn't work in my environment.
2. Add a web resource to retrieve related notes and render data by ourselves.
It is very straightforward, I built a demo, it uses Bootstrap, jQuery and pagination JS, the custom attachment manager will show 5 records per page.
To test, I uploaded 6 notes to my contact.


You could run it in your environment to test.
Title
3. Import a custom control(PCF component) to show notes as subgrid, but I didn't find a perfect control on existing projects of PCF Gallery.
In a word, the best solution might be building a custom control by yourself.
Regards,
Clofly