get event by ID: no RELATEDTOID nor SEMODULE

get event by ID: no RELATEDTOID nor SEMODULE

Hello!
We are facing strange problem: when we look at the event on UI we see
that it is attached to a Lead. But when we load this event over API
then response does not contain reference to the Lead (no RELATEDTOID nor SEMODULE fields are present in response).
Could you please, explain why is this happening?
thanks,
Anton