I also experienced some initial side effects from NAC by excessive slime secretion in the respiratory system, which I think is typical for high doses of NAC, but when a deficiency is balanced out, even smaller amounts might lead to similar effects.
I wonder if the quality of NAC also plays a role. I noticed that my Bulk NAC smells too sulfuric and I suspect the lack of encapsulation increased the oxidation of it. Fresh NAC smells like vinegar, as far as I know and given my experience with different NAC products. Unfortunately, NAC has become restricted as you know, and my favorite manufacturer, Jarrow, took their sustained NAC off market. I asked them if they intend to relist it eventually and they responded that it's not going to happen anytime soon and that everything depends on the FDA now.
TLDR yet:
Stability of N-Acetylcysteine (NAC) in Standardized
Pediatric Parenteral Nutrition and Evaluation
of N,N-Diacetylcystine (DAC) Formation (2020) [doi:
10.3390/nu12061849]