I do remember trying to wash a PCB out of an audio amplifier in a dishwasher once after I had read this method was used in the Tektronix labs to reduce leakage on the board. It did not work...
They used something which I found later to have been called "Kelite", but I have not been able to find a source for it, just a bunch of people that also did not have it.
Using ordinary dishwasher blocks (wihout salt and shine) all the flux residue on the board turned white and did not wash off, causing it to look similar to your Kasuntest. One would needed to scratch it off my PCB with a finger nail rather than wiping it, but it might still be similar stuff.
On a Dutch forum I found people that had PCBs that turned white in a similar way due to water ingress. It seems like water turns flux residue white, but only on the surface, because wettening the affected areas with isopropyl alcohol immediately removed the white effect and showed the brown colour underneath.