@pawan.singh Thanks for your patience on this.
I have checked with Azure Communication Service Engineering on this and sharing the below.
Chat Service in ACS is inherently different from calling Service and also it is a store and forward system and doesn't require a live connection for messages.
Once the participant is added to a thread it will be there until explicitly removed and has access to the messages on the thread regardless of the connection status.
Feel free to reach back to me if you have any further questions on this.