mjhjmtu

I'm trying to debug an issue with the man Neovim plugin.

At :h ft-man-plugin, Neovim recommends to export this variable to use it as a manpager:

export MANPAGER='nvim +Man!'

But if I execute $ man man from the shell, then :Man mount from Neovim, the latter raises this error:

man.vim: command error (11) man -w mount: No manual entry for mount
See 'man 7 undocumented' for help when manual pages are not available.

I've been able to reproduce the issue with this minimal example:

$ MANPAGER='vim -Nu NONE -' man man
:!man -w mount

The first shell command starts the $ man command, using Vim as a manpager (although, here, it's not configured correctly to display the page).
The second Vim command asks a shell to execute $ man -w mount to find the location of the page for the $ mount command. Here is its output:

No manual entry for mount
See 'man 7 undocumented' for help when manual pages are not available.

shell returned 16

I've also tried with Neovim instead of Vim, and with system() instead of :! (i.e. :echo system('man -w mount')), but the result is always the same; $ man doesn't find the page for the $ mount command.

The issue disappears if (Neo)Vim is started directly with the $ vim (or $ nvim) command instead of indirectly via $MANPAGER.
So these commands correctly find the location of the $ mount command:

$ zcat /usr/share/man/man1/man.1.gz | vim -Nu NONE -
:!man -w mount

It also disappears if I ask for a page from the first section of the manual.
So these commands correctly find the location of the $ ls command:

$ MANPAGER='vim -Nu NONE -' man man
:!man -w ls

It also disappears if I manually provide the section number of the page.
So these commands correctly find the location of the $ mount command:

$ MANPAGER='vim -Nu NONE -' man man
:!man -s8 -w mount

The issue happens only when (Neo)Vim is started as a manpager and I ask $ man to find a page outside the first section of the manual, without providing the section number of the page.

Here are some information about my machine:

• 
  vim --version: VIM - Vi IMproved 8.1 Included patches: 1-495


• Operating system: Ubuntu 16.04.5 LTS


• Terminal emulator: rxvt-unicode v9.22


• Terminal multiplexer: tmux 2.8


• 
  $TERM: tmux-256color


• Shell: zsh 5.6.2-dev-0 (x86_64-pc-linux-gnu)

Why can't $ man find the pages outside the first section when Vim was started as a manpager?