I now suspect that when the model was built, there was an “excursion” of the tap water. With civil water systems, this can occur for a number of reasons, including but not limited to cleaning operations, maintenance, and new methods.
Bottom line is: use distilled water to dilute aqueous acrylics. I don’t even use tap water for cleaning. (Our water is saturated with calcium and magnesium carbonates.)
I also like the idea of a thread on care and maintenance of built kits.