Often times, in some scenarios, the display on the expansion modules (D3 / D7) may display unintended anomalies (e.g. vertical lines or wrong texture). This unintended behavior can be resolved with a firmware upgrade of your D3 or D7 to version 2.1.1 or higher.
We also recommend to use the latest firmware version on your desktop phone.
If the issue is still present after the upgrade, please, open a support ticket.