:Maps in preview popup window are not highlighted properly on Mac OS X Catalina

- [ x ] I have read through https://github.com/junegunn/fzf.vim/blob/master/README.md
- [ x ] I have read through https://github.com/junegunn/fzf/blob/master/README-VIM.md
- [ x ] I have read through the manual page of fzf (`man fzf`)
- [ x ] I have searched through the existing issues

Here is the screenshot of `:Maps` running on my Mac (the prompt `Maps (n)` was not highlighted properly):

This problem persists on the minimum config. Here is another screenshot of `:FzfHistory:` on the same machine (the prompt `Hist:>` was highlighted properly:

Here are my options:
```bash
export "FZF_DEFAULT_OPTS"="--layout=reverse --exact --ansi --color=dark,hl:9,hl+:136,bg+:0,fg+:14,info:5,prompt:33,spinner:108,pointer:226,marker:235,header:33"
```

For `init.vim`:
```vim
let g:fzf_preview_window = 'right:50%'
let g:fzf_layout = { 'window': { 'width': 0.9, 'height': 0.9 } }
```

Neovim version:
```bash
NVIM v0.4.3
Build type: Release
LuaJIT 2.0.5
Compilation: /usr/local/Homebrew/Library/Homebrew/shims/mac/super/clang -U_FORTIFY_SOURCE -D_FORTIFY_SOURCE=1 -DNDEBUG -DMIN_LOG_LEVEL=3 -Wall -Wextra -pedantic -Wno-unused-parameter -Wstrict-prototypes -std=gnu99 -Wshadow -Wconversion -Wmissing-prototypes -Wimplicit-fallthrough -Wvla -fstack-protector-strong -fdiagnostics-color=auto -DINCLUDE_GENERATED_DECLARATIONS -D_GNU_SOURCE -DNVIM_MSGPACK_HAS_FLOAT32 -DNVIM_UNIBI_HAS_VAR_FROM -I/tmp/neovim-20191107-85913-1wbgqz6/neovim-0.4.3/build/config -I/tmp/neovim-20191107-85913-1wbgqz6/neovim-0.4.3/src -I/usr/local/include -I/tmp/neovim-20191107-85913-1wbgqz6/neovim-0.4.3/deps-build/include -I/usr/local/opt/gettext/include -I/Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX10.15.sdk/usr/include -I/tmp/neovim-20191107-85913-1wbgqz6/neovim-0.4.3/build/src/nvim/auto -I/tmp/neovim-20191107-85913-1wbgqz6/neovim-0.4.3/build/include
Compiled by brew@Catalina.local

Features: +acl +iconv +tui
See ":help feature-compile"

system vimrc file: "$VIM/sysinit.vim"
fall-back for $VIM: "/usr/local/Cellar/neovim/0.4.3/share/nvim"

Run :checkhealth for more info
```