case / international 585 farm tractor?

I know very little about tractors, and even less about what you are asking us to help you with.
Am I correct in assuming that normally a tractor won't start if it's left in gear, and yours is faulty because it will?
This being the case, my guess is there will be a switch or sensor somewhere on the gearbox housing or the neutral zone on the gear selector that's wired to the ignition system. See if you can locate anything in these areas and look for a damaged cable that's grounding out and causing the issue. If all the wiring looks good you will probably have to withdraw the switch or sensor to test it and replace if necessary.
As I say, this reply is just a suggestion based on guesswork as I have no expert knowledge or experience, but would seem logical that it would be something liike that.
Good luck and let us know what the fix actually is for future reference.

Rupert
June 2020