This behavior is documented in Cisco bug ID CSCdy58717.
TCP/UDP connections do not time out. This prevents translation (xlate) entries from timing out as well. Issue these commands in order to check whether connections do not time out:
The connection timeout value must not be larger than the timeout value for the idle connections.
As a workaround, perform either of these two tasks:
As an alternative, download and upgrade the software version to the latest available version.