The oxidation in solution of phosphorous acid is too slow to worry about a document I read stated. Very very minimal.
But when it's in the open air, and with heat it'll happen rapidly. But then, if it's left out in the open it'll form it's own solution and the process will slow down as that happens.
There's nothing to worry about, it'll get you high for quite a few years if that's the concern, it just might need to be used with a bit more excess as time goes by.
But if you've been extracting it from foliar solutions crudely, then hopefully you'll start using vacuum for it's drying...which will more than counter balance the oxidation issue as far as level of quality.
The potassium phosphite salts also oxidize, at a similar rate, in to phosphate.