Thanks, @Airy.
Peter's first post in that thread also states that KM "uses double to store values internally" -- so I guess the question is "at what point do they become 'internal', and for how long?".
Which is why I'm starting to doubt my own sanity 
But it's reproducible, even within a calculation (so probably not a string->number conversion issue):
At which point I have to assume problems at my end -- or problems with me!