Does this happen if the RDP session has been idle for ten minutes? The firewall/NAT function will by default tear down sessions with no traffic after a timeout period. This prevents hung sessions from tying up resources indefinitely. The default is 600 seconds (ten minutes).
Try the following commands:
ip policy-timeout tcp 3389 7200
ip policy-timeout udp 3389 7200
This will keep idle RDP sessions alive for two hours, which should be sufficient in most cases. You can make it longer if you desire.
unfortunately RDP sessions disconnect regardless of being idle or not. sometimes a user would be working in the RDP session and it would disconnect right in the middle of an action.