Well, to be fair its datasheet mentions the need to clean after use, so not necessarily surprising.
https://mgchemicals.com/downloads/tds/tds-8341.pdf
Overall I found it cleans much easier than the other types I have used, so that is a big plus. Also, for anything high impedance or high voltage I would not rely on leaving residue on the PCB anyways, but YMMV.
The problem is that even when you clean the boards (I actually wash mine pretty thouroughly by immersing them in IPA, letting them soak, then cleaning with a brush), there is no guarantee that some residue will not remain under e.g. SMD parts or other components that sit flush against the board so that the brush can't reach under them.
IPA only dilutes the flux, or makes it thinner, so that it becomes easier to remove it mechanically, but it doesn't dissolve it in the actual sense of the word -- so if you just soak the board in IPA, let it sit there, then pull it out and let it air dry, then after drying the flux will almost all be there.
So even with cleaning, for the circuits where the fluxes' conductivity, even if very low, may matter, I would still prefer a flux whose conductivity is below the sensitivity of a megohmmeter. At least for hand soldering -- it is said that when the "no-clean" fluxes are heated in a reflow oven to a certain temperature, they lose conductivity. I cannot verify this.
So, back to the 8341. I have now also soldered some stuff with it and compared it with Relife RL-422-IM and Mechanic 225 that I've also used recently.
- very thick -- a 16 gauge needle with a 5ml syringe is barely enough; also rather tricky to transfer it from the jar to a syringe
- wetting properties are good, better than RL-422-IM, but slightly worse than Mechanic 225
- slightly more fumes and more smelly than RL-422-IM, but much, much more less so than Mechanic 225
- does not boil out or lose its properties as fast as the Relife (which is rather poor in this aspect), stays in place really nicely, so it requires much less frequent reapplication. I think it's actually better in this than the Mechanic, too. This isn't yet a final observation, as I have to do more soldering: I'll soon have a PCB with a few dozens of SMD and a few THT components to solder, and that's going to be a good test.
- very easy to clean indeed: doesn't need a prolonged soaking in IPA, doesn't leave a white powder-like residue in hard to reach spots (the Mechanic does this). The Relife flux is similar, but maybe a little harder to clean, and the Mechanic is quite stubborn (and is a bitch to clean when it gets onto your hands).
Now, I must admit that, with the previous fluxes, before the 8431, I was doing my conductivity measurements wrong! I used the wrong terminal for the negative probe on the megohmmeter, and, while it did show the breakdown voltage (I think), it didn't show the resistance. D'oh! Shame on me. For example, I've now measured the Mechanic, and it does conduct, and it seems to be actually worse than the 8431.
I did, however, use the nanosiemens range on the BM869s correctly, and not all fluxes showed conductivity in that case.
I will make a small PCB consisting of two opposing polygons of a known size separated by a gap of a known width and redo the measurements of conductivity of all the fluxes I have in a controlled and reproducible manner. Just shoving two probes into a blob of flux and measuring if it conducts at an uncontrolled distance between the tips isn't good enough at this point.