Back the nut off, put a little orange loctite on it.

I'll try some locktite but am really struggling to understand why it is loosening in the first place.  I noticed that the washer with integrated O-ring was missing the rubber portion and am not sure if that was a cause of the leak which then made the bolt back out easily or the effect of continuously trying to tighten the nut to then have it come loose again.  Either way, I replaced the washer yesterday and will see if that works.  I looked for thread locker at that time but couldn't find it but will be adding some next time for sure