Why does Li get darker in a glove box? I'm having some issues with lithium getting darker in a glove box. Please see attached photos, Li gets dark overnight.

My sensors for $\ce{H2O}$ and $\ce{O2}$ are showing 0 ppm. I also checked oxygen and moisture contents using chemicals in case sensors were bad, and looks like there is no oxygen and moisture in the glove box. Just to make sure, I also replaced the nitrogen tank with argon (there are some reports in the literature saying Li reacts with nitrogen even at room temperature and gets darker in the presence of trace levels of moisture).

Does anyone had this problem before? Any recommendations would be of great help.
Lithium can react with nitrogen giving a dark product.  https://en.wikipedia.org/wiki/Lithium