I have worked with NSI for nearly 2 decades, I have also been fortunate enough to be part of the testing team for many products. I have never in all my years seen this happen unless the product was contaminated as already mentioned.
I would say that someone has accidentally contaminated your monomer bottle with something, possibly acetone.
I have seen/been given products to test that were apparently out of date to see how they performed compared to new products and again, I never saw a problem like this.
Another thought, are you using the same brush for every different system you are using? I know many people think that monomer is monomer, powder is powder etc but I can assure you that for the companies that make their own products, all these products are made to chemically work together, so when you add another brand, you can not guarantee that the system will work the same.
I once helped a new tech who tried many systems and after a couple of trials, things started to get slowly worse, I finally realised she was using the same brush for each system, so her brush was being used in many different chemical baths and this caused problems with the brush, I got her to use different brushes for each system she wanted to use, all her problems vanished straight away.
I say this story to help with understanding that every system is different (providing it's a system made by the company itself and not a mass produced system that can be bought and then anybody can put their name to it) and it's always best to use every item incl brush from that system to get best results.
Get a new set of products, get a new brush and make sure you only use these products together and I think all your problems will be solved. I will be very surprised if they are not.