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The problem you describe is familiar, though it doesn't happen often.
It might be the autopilot module itself, that directs everything, is defective, and its simply insisting on right turns without reason.
Or it might be, that corrosion (always a hazard in maritime or riparian circumstances) has gotten into one of the connections that leads to the autopilot, and bad feedback on position and/or commands and/or relative direction causes the steering to the right.
So, take apart connections that lead to the autopilot, from
- the GPS antenna
- and/or the autocompass (especially, since it indicates direction)
- and the motor that moves the boats steering mechanism(s)
...take apart each connector, one at a time, and scrub both sides of the connection with a clean dry brush, then wipe down with a clean dry rag cloth until the metal shines, then re-connect firmly, then on to the next connector, and do the same scrub-and-wipe-and-re-connect procedure, etc.
Try again. If it still acts wrong, then check the autocompass reading any way you can to make sure it isn't locked into a wrong reading... doing a compass check circular turn while watching the compass reading, for instance, can check on that.
Then after that, if nothing else is found wrong, the autopilot itself has suffered a (admittedly somewhat rare) malfunction, most likely. At least, that would be my guess, and it would be a reasonable thing to try replacing first, given that the autocompass is still demonstrably not jammed nor indicating wrongly.