Hi @Shawn_Varughese and welcome in the MongoDB Community 
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Are you creating a new connection (==mongoClient) for each Lambda or your MongoDB Connections are shared across multiple (all) lambdas?
I give you a little hint, “yes” is a very bad answer 
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Check out this doc where they explain how to set up the connection correctly:
Using a private endpoint would also be a plus (instead of 0.0.0.0/0).
Cheers,
Maxime.