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There are two Autopilots/displays in the system, then.
When the Helm ap/disp. goes back to Standby, is the 'upper' ap/disp. in Standby? if they are both connected up to the same drive motor, then one being in Auto would probably mean the other would recognize it as already in-control, and go back to Standby.
If the going-to-Standby-at-once habit of the Helm ap/disp. happens even when the upper unit is in Standby, then you have another sort of problem. If that is the case, check all wiring from the Helm unit to the drive motor (and through any other modules) for breaks, severe bends, cuts, etc.
Then turn Off the system, and disconnect the connectors on the way to the drive motor from the Helm ap/disp. unit, one at a time, and scrub both sides of the connector with a clean dry brush for 20+ seconds, then wipe down both sides of the connector with a clean dry rag cloth, then re-connect again, firmly, and on to the next in the line to the drive motor.
Then try the system again, and the Helm autopilot/display in Auto.