Even though Session Roaming was disabled for customer’s Citrix environment, users were ‘hijacking’ their Citrix sessions randomly when launching applications from two separate computers. These users had recently been migrated to XenApp 6.5 environment using Storefront (from XenApp 6.0\Web Interface configuration).
Troubleshooting showed that the hijacking was only occurring for the user when Citrix load evaluators placed the user on the same Citrix server in the farm for both sessions. The issue did not have to do with the Citrix Session Roaming feature, but rather an RDS setting to limit users to only one session per RDS server.
The resolution is to modify RDS Host Configuration setting to not ‘Restrict each user to a single session’. This setting is configured on each individual RDS\Citrix server.