Does TrueCalller App leak your phonebook? Sometimes, I cannot find new contacts that I add to Truecaller phonebook, but sometimes, I think if it shares/leaks my contacts to public. 
It shows other people contact details that I never added there. 
So I am thinking if it is doing the same to me. 

Andoird: 7.1.1
Phone: Oneplus 3T
Yes! it does, when using Truecaller application or services you have already accepted their :

Truecaller - Privacy Policy 

So, in this case you agree that :

Truecaller may use your personal information collected by their application to provide, maintain, improve, analyze and personalize their Services for other Users, partners and third party providers. 

More specifically, Truecaller may use such information to provide names of contacts from your address book to other Trucaller users.