I would do:

"italic syntax highlighting autocmd FileType cpp,hpp syntax match Italic "\v<_[a-zA-Z0-9_]*_>" containedin=cComment,cCommentL contained autocmd FileType cpp,hpp syntax match ConcealUnderscore /_/ containedin=Italic contained conceal highlight Italic cterm=italic gui=italic ctermfg=lightcyan guifg=lightcyan autocmd FileType cpp,hpp setlocal conceallevel=2 concealcursor=nc 

I would do:

"italic syntax highlighting autocmd FileType cpp,hpp syntax match Italic "\v<_[a-zA-Z0-9_]*_>" containedin=cComment,cCommentL contained autocmd FileType cpp,hpp syntax match ConcealUnderscore /_/ containedin=Italic contained conceal highlight Italic cterm=italic gui=italic ctermfg=lightcyan guifg=lightcyan autocmd FileType cpp,hpp setlocal conceallevel=2 concealcursor=nc 

The conceallevel of the netrw buffer is 0 and so is by default the conceallevel of the buffer open from it. That is why you have to ensure it is set correctly.