Steps to reproduce

1. export MANPAGER=vim +MANPAGER --not-a-term -
2. man man
3. Vim opens with the man page for 'man'
4. :Man ls

Expected behaviour

It should open a new empty buffer in the same window and read in the manpage for ls

I've printed out the command that is being executed and did some debugging:
env -u MANPAGER GROFF_NO_SGR=1 man 'ls' | col -b

1. removing silent shows "shell returned 1"
2. removing | col -b and reading the command in displays the contents of the man page for ls

Version of Vim

VIM - Vi IMproved 9.0 (2022 Jun 28, compiled Apr 24 2023 18:31:22) Included patches: 1-1486

Environment

Linux 5.15.102-1-pve #1 SMP PVE 5.15.102-1 (2023-03-14T13:48Z) x86_64 GNU/Linux
$SHELL = zsh 5.8 (x86_64-debian-linux-gnu)
Terminal = iTerm2
$TERM = screen-256color

VIM - Vi IMproved 9.0 (2022 Jun 28, compiled Apr 24 2023 18:31:22)
Included patches: 1-1486
Compiled by tart@lemontart
Huge version without GUI. Features included (+) or not (-):
+acl +file_in_path +mouse_urxvt -tag_any_white
+arabic +find_in_path +mouse_xterm -tcl
+autocmd +float +multi_byte +termguicolors
+autochdir +folding +multi_lang +terminal
-autoservername -footer -mzscheme +terminfo
-balloon_eval +fork() +netbeans_intg +termresponse
+balloon_eval_term -gettext +num64 +textobjects
-browse -hangul_input +packages +textprop
++builtin_terms +iconv +path_extra +timers
+byte_offset +insert_expand -perl +title
+channel +ipv6 +persistent_undo -toolbar
+cindent +job +popupwin +user_commands
-clientserver +jumplist +postscript +vartabs
-clipboard +keymap +printer +vertsplit
+cmdline_compl +lambda +profile +vim9script
+cmdline_hist +langmap -python +viminfo
+cmdline_info +libcall -python3 +virtualedit
+comments +linebreak +quickfix +visual
+conceal +lispindent +reltime +visualextra
+cryptv +listcmds +rightleft +vreplace
+cscope +localmap -ruby +wildignore
+cursorbind -lua +scrollbind +wildmenu
+cursorshape +menu +signs +windows
+dialog_con +mksession +smartindent +writebackup
+diff +modify_fname -sodium -X11
+digraphs +mouse -sound -xfontset
-dnd -mouseshape +spell -xim
-ebcdic +mouse_dec +startuptime -xpm
+emacs_tags -mouse_gpm +statusline -xsmp
+eval -mouse_jsbterm -sun_workshop -xterm_clipboard
+ex_extra +mouse_netterm +syntax -xterm_save
+extra_search +mouse_sgr +tag_binary
-farsi -mouse_sysmouse -tag_old_static
system vimrc file: "$VIM/vimrc"
user vimrc file: "$HOME/.vimrc"
2nd user vimrc file: "~/.vim/vimrc"
user exrc file: "$HOME/.exrc"
defaults file: "$VIMRUNTIME/defaults.vim"
fall-back for $VIM: "/usr/local/share/vim"
Compilation: gcc -c -I. -Iproto -DHAVE_CONFIG_H -O2 -fno-strength-reduce -Wall -Wno-deprecated-declarations -D_REENTRANT -U_FORTIFY_SOURCE -D_FORTIFY_SOURCE=1
Linking: gcc -L/usr/local/lib -Wl,--as-needed -o vim -lm -ltinfo -lrt -ldl

Logs and stack traces

No response