New Certificate Token is not set after 45min of OpcUa Client/Server communicate|OPC UA Implementation: Stacks, Tools, and Samples|Forum|OPC Foundation

Hello,
I am using .net SDK from https://github.com/OPCFoundati.....ETStandard to develop OPCUA client. 
My client is communicating to a remote OpcUa Server. Using BasicSHA256 certificate to communicate.
It connects and reads a variable continuously without any problem. Initial hand-shake is fine with certificate exchange. I see that public/private keys are present in the beginning with client certificate.
Exactly after 45min, the connection fails with a message "SecureChannelClosed". (bcos TokenRenewalPeriod = 0.75 and DefaultSecurityTokenLifeTime = 3600000 are set at client and SDK resp.)
I debugged SDK to find that "privatekey" of the client certificate is shown as "NULL" at this token exchange process.(GetRSAPrivateKey returns null at Rsa_Sign().)

I could not understand how this token is updated from client end after 45min of communication.
Please help me.

with regards,

Hemanth.