supertab insert mapping does not exist after installing supertab

[autumn-wind]

"No mapping found" is shown after I type "verbose imap ".
Confirmed supertab is loaded by typing ":scriptnames" -- "70: ~/.vim/plugged/supertab/plugin/supertab.vim"

I'm in a docker container:
`➜ ~ cat /etc/os-release
NAME="CentOS Linux"
VERSION="7 (Core)"
ID="centos"
ID_LIKE="rhel fedora"
VERSION_ID="7"
PRETTY_NAME="CentOS Linux 7 (Core)"
ANSI_COLOR="0;31"
CPE_NAME="cpe:/o:centos:centos:7"
HOME_URL="https://www.centos.org/"
BUG_REPORT_URL="https://bugs.centos.org/"

CENTOS_MANTISBT_PROJECT="CentOS-7"
CENTOS_MANTISBT_PROJECT_VERSION="7"
REDHAT_SUPPORT_PRODUCT="centos"
REDHAT_SUPPORT_PRODUCT_VERSION="7"`

➜ ~ vim --version VIM - Vi IMproved 7.4 (2013 Aug 10, compiled Dec 15 2020 16:44:08) Included patches: 1-207, 209-629 Modified by <bugzilla@redhat.com> Compiled by <bugzilla@redhat.com> Huge version without GUI. Features included (+) or not (-): +acl +farsi +mouse_netterm +syntax +arabic +file_in_path +mouse_sgr +tag_binary +autocmd +find_in_path -mouse_sysmouse +tag_old_static -balloon_eval +float +mouse_urxvt -tag_any_white -browse +folding +mouse_xterm -tcl ++builtin_terms -footer +multi_byte +terminfo +byte_offset +fork() +multi_lang +termresponse +cindent +gettext -mzscheme +textobjects -clientserver -hangul_input +netbeans_intg +title -clipboard +iconv +path_extra -toolbar +cmdline_compl +insert_expand +perl +user_commands +cmdline_hist +jumplist +persistent_undo +vertsplit +cmdline_info +keymap +postscript +virtualedit +comments +langmap +printer +visual +conceal +libcall +profile +visualextra +cryptv +linebreak +python/dyn +viminfo +cscope +lispindent -python3 +vreplace +cursorbind +listcmds +quickfix +wildignore +cursorshape +localmap +reltime +wildmenu +dialog_con -lua +rightleft +windows +diff +menu +ruby/dyn +writebackup +digraphs +mksession +scrollbind -X11 -dnd +modify_fname +signs -xfontset -ebcdic +mouse +smartindent -xim +emacs_tags -mouseshape -sniff -xsmp +eval +mouse_dec +startuptime -xterm_clipboard +ex_extra +mouse_gpm +statusline -xterm_save +extra_search -mouse_jsbterm -sun_workshop -xpm system vimrc file: "/etc/vimrc" user vimrc file: "$HOME/.vimrc" 2nd user vimrc file: "~/.vim/vimrc" user exrc file: "$HOME/.exrc" fall-back for $VIM: "/etc" f-b for $VIMRUNTIME: "/usr/share/vim/vim74" Compilation: gcc -c -I. -Iproto -DHAVE_CONFIG_H -O2 -g -pipe -Wall -fexceptions -fstack-protector-strong --param=ssp-buffer-size=4 -grecord-gcc-switches -m64 -mtune=generic -D_GNU_SOURCE -D_FILE_OFFSET_BITS=64 -D__linux__ -D_REENTRANT -U_FORTIFY_SOURCE -D_FORTIFY_SOURCE=1 Linking: gcc -L. -Wl,-z,relro -fstack-protector -rdynamic -Wl,-export-dynamic -Wl,--enable-new-dtags -Wl,-rpath,/usr/lib64/perl5/CORE -Wl,-z,relro -L/usr/local/lib -Wl,--as-needed -o vim -lm -lnsl -lselinux -lncurses -lacl -lattr -lgpm -ldl -Wl,--enable-new-dtags -Wl,-rpath,/usr/lib64/perl5/CORE -fstack-protector -L/usr/lib64/perl5/CORE -lperl -lresolv -lnsl -ldl -lm -lcrypt -lutil -lpthread -lc ➜ ~

[autumn-wind]

And I use https://github.com/junegunn/vim-plug as the vim plugin manager to install supertab

`" Specify a directory for plugins (for Neovim: ~/.local/share/nvim/plugged)

call plug#begin('~/.vim/plugged')

" Make sure you use single quotes

Plug 'scrooloose/nerdtree'
Plug 'scrooloose/nerdcommenter'
Plug 'kien/rainbow_parentheses.vim'
Plug 'vim-airline/vim-airline'
Plug 'vim-airline/vim-airline-themes'
Plug 'tomasr/molokai'
"Plug 'Shougo/neocomplete.vim'
Plug 'vim-syntastic/syntastic'
Plug 'spf13/vim-autoclose'
Plug 'Raimondi/delimitMate'
Plug 'tpope/vim-surround'
Plug 'majutsushi/tagbar'
Plug 'easymotion/vim-easymotion'
Plug 'rking/ag.vim'
Plug 'thinca/vim-quickrun'
Plug 'junegunn/fzf', { 'dir': '~/.fzf', 'do': './install --all' }
Plug 'AndrewRadev/splitjoin.vim'
"Plug 'SirVer/ultisnips'
Plug 'terryma/vim-multiple-cursors'
Plug 'ctrlpvim/ctrlp.vim'
Plug 'derekwyatt/vim-fswitch'
Plug 'davidhalter/jedi-vim'
Plug 'ervandew/supertab'
"Plug 'fatih/vim-go', { 'do': ':GoUpdateBinaries' }

" Initialize plugin system
call plug#end()`

[autumn-wind]

after building vim from source to version 9.1, this issue disappear but still don't know why.