Consider the following script:

% vim -u NONE -i NONE -N --cmd 'nnoremap <C-u> x' --cmd 'set isident=' --cmd 'nnoremap <C-x> u' --cmd $'nnoremap \C-x|nnoremap \C-u|echo "-"|nnoremap' --cmd cq
No mapping found
n  <C-U>       * x
-
n  <C-U>       * x
n  <C-x>       * u

You may see here that while <C-u> mapping was defined as expected <C-x> mapping was defined exactly as if I used <LT>C-x>, which is not the case. I do not really see why &isident option needs to affect VimL parsing at all (I mean parsing $env, this does not include /\i and “after a match of the 'define' option”: $env is editor-global, /\i and &define could be/are filetype-specific), but this specific case is listed neither under :h 'isident' nor under :h key-notation.

The root of the problem is the use of vim_isIDc() in find_special_key().

Vim version 8.0.1358.