It seems the issue comes from the \! after \not - I don't know why. If I remove it, it compiles ok. But not symbol is no more centered.
You would need to search in the LaTeX world why this cause an issue in beamer, and see if you can find another way to write the not independant symbol.
This is a LaTeX issue unfortunately - I don't think we can do much in R for that.
Thanks for looking into it, indeed centering is exactly what I was trying to achieve. I will follow your advice and see if I can find a solution through the LaTeX community.