A curious thing was happening when users entered the TEST mode in the RCFORB software. Pressing the TEST button switched the audio input to the IC-7200 from USB to MIC, which meant that remote users could then not modulate when transmitting in SSB mode. CW was not affected.
The solution was to remove the TEST button from the client interface in the security settings on the host software. Once that change was made, no instances of spontaneous switching from USB to MIC have occurred.