Need to keep re-soldering SMD resistors on PCB
This seems to be plumbing flux, which often is higher in corrosive components than electronics fluxes. Considering solder wire comes with a rosin core, you'll probably be better off without that flux, as such fluxes have the tendency to corrode electronic contacts over time.

I've never head of that happening within weeks, however.

So, my hypothesis is: 


you got a "cold" solder join, meaning that there's no wetting of the contacts on both sides. (you can google image search for "cold solder joint")
You have a large capacitance on the OutCh1 side that you don't show. After having charged that to a specific voltage, it takes long to discharge it (depends on how shortly after changing InCH1 you look)
Whatever you have on the output actually latches up.