Thanks everyone for the input, especially ajk for that link to the thread with the additional Star San info.
In this case of an immediate, drastic change in the time-to-clouding observed from one bottle of SS to the next, all I can suppose there is some appreciable difference between the two batches of Star San that I own. How and why they came to be different is unknown.
What I do know now is that I need to change my usage practices. I'll immediately start with the suggestion of using distilled water to mix up solutions that go into my spray bottles or in any situation in which I wish to keep the mix beyond a day.
Cheers and happy sanitizing!