Incremental Search binding does not work

[ed9w2in6]

For some reason the Incremental Search binding <C-S> does not work.
Backwards searching <C-R> works though.

Pretty much every other bindings that I know so far is working.

I am using macOS.
I tested this on both iTerm2.app and Terminal.app

I am pretty sure I do not have terminal emulator or OS keybinds that intercepts <C-S>

System Info:

VIM - Vi IMproved 8.1 (2018 May 18, compiled Jul 5 2020 01:22:14)
macOS 10.15.6（19G73) #i.e. Catalina
Vimacs (0.96)

Running :verbose imap <C-S>

i  <C-S>       * <C-O>:call <SNR>8_StartSearch('/')<CR><C-O>/
      Last set from ~/.vim/plugin/vimacs.vim line 385

Edit 2020-07-19:
At Vimacs version 0.96, the keybind <C-X><C-S> or <C-X><C-C> did not work as well.

Rolling back to Vimacs version 0.93 fixed this.
Incremental Search binding <C-S> still does not work though.