Leucine and valine don't have much solution chemistry on their own.  I
suspect the reason for refrigeration is simply to keep microbial growth down
as a result of contamination from opening/closing, etc. since the solvent
isn't preserved.  In the case of antibiotic suspensions, refrigeration is
required to slow down breakdown of the drug molecule (e.g., hydrolysis of
beta-lactams).  Some like erythromycin is sufficiently water insoluble to
survive a real long time in the suspension.

And then, sometimes, you'll find recipes that state storage conditions and
expiry that are completely arbitrary.  Just because some guy thought it was
a good idea.