This is more than true. Once the water has nutrition in it for bacteria to thrive on its game time. Lots of nutrient lines come with artificial sugars like dextrose mixed in which will attract bacteria and breed it fast.
I personally mix only what I need anymore, so for example if you know you won't need 20 gallons maybe mix a 5 gallon batch and mark your bucket so that you can use it up without having it sitting and stagnating.
Good luck, it's always something!
I did a quick look around and couldn't get a definitive list of ingredients in the CannaCoco A+B 'system.' All very hush hush.
I do see they list fulvic and humic, so I'd give it a 50/50 that there's some other organics involved that are creating a great space for microbe growth.
But if you do read their copy, they talk about how it needs to be home mixed for use because otherwise it gets slimy...that would be some kind of tip off to me that it's not super happy to sit around. There's could also be other factors involved like if the rez is open or sealed, if there's light getting to the water or not. We've already established that there's no air stone involved.
I do mix up my nutrient/water base a day or two ahead of time (to dissolve the powder), but it's all salt based and it's in 1g jugs that I cap right off. Any organics involved, I put in minutes before watering and any of that doesn't stay around if there are leftovers...right down the drain.