That sounds like it's the likely cause; SniffingConnectionPool seeds itself on startup and at intervals with known nodes in the cluster, based on what is returned from /_nodes/http,settings API call.
Do nodes have TLS configured for the HTTP layer? Would it be possible to capture what is returned from the first request with .DisableDirectStreaming() set on ConnectionSettings? It sounds like the client may not be handling this gracefully so any further detail that you can provide will help.
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