Keytroller has been selling and servicing the Vanguard weighing system for more than 5 years. The system itself is galvanicly isolated against noise.

There have been certain trucks that require further isolation. These are typically AC powered trucks. Keytroller has a non-conductive fitting that further isolates the solenoid, transmitter and the transducer.
In most cases this corrects the issue.

The only explanation I have for the new behavior after 2 years of UN-eventful service is possibly some electrical part wearing beyond tolerance.
Such as a drive motor or other electrical part that has worn brushes. The brushes are chattering and emitting more electrical noise than normal.
The Vanguard suddenly picking up this noise through cabling or hydraulic hose and transmitting it though the bypass module and causing the solenoid to energize.
We have worked with this dealer/customer on this issue, and have recommend the non-conductive fitting as described above.