I have read that the driver's side cold, passenger side warm is some sort of fail safe to ensure the driver gets the cooling as long as possible when there is a problem until it fails completely. There was a long discussion on this on a board a few years ago. I know the last time I had it happen a quick charge cured the problem and no leaks were found so I guess it may have been a SP seal.
It's so darned hot at present I hope you find the solution as cheaply as possible.

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