Maybe there's some difference in our environment variables?

  debtest$ printenv
  _=*4625*/usr/bin/printenv
  CLUTTER_BACKEND=x11
  COLUMNS=80
  CPU=x86_64
  DBUS_SESSION_BUS_ADDRESS=unix:path=/run/user/1000/bus
  DESKTOP_SESSION=mate
  DISPLAY=:0
  DMAKE_OUTPUT_MODE=TXT2
  EDITOR=emacsclient
  GDMSESSION=mate
  GIT_PAGER=
  GPG_AGENT_INFO=/run/user/1000/gnupg/S.gpg-agent:0:1
  GS_OPTIONS=-sPAPERSIZE=letter
  GTK_MODULES=xapp-gtk3-module:gail:atk-bridge:canberra-gtk-module
  GTK_OVERLAY_SCROLLING=0
  HGRCPATH=/home/kupfer/.hgrc:/home/kupfer/.hg.d
  HOME=/home/kupfer
  HOST=debtest
  IGNOREEOF=4
  INSIDE_EMACS=27.1,comint
  LANG=en_US.UTF-8
  LC_TIME=C
  LOGNAME=kupfer
  MAILCHECK=60
  MANPATH=/home/kupfer/man:/usr/local/man:/usr/local/nmh/share/man:/usr/share/man:/usr/X11R6/man
  MATE_DESKTOP_SESSION_ID=this-is-deprecated
  MDK_PROFILE_DONE=Mon Jun 14 07:26:07 PDT 2021
  OS=Linux
  PATH=/sbin:/usr/sbin:/home/kupfer/bin:/home/kupfer/bin/x86_64:/usr/local/bin:/usr/bin/mh:/usr/bin:/bin:/usr/X11R6/bin
  PS1=debtest$ 
  PWD=/home/kupfer
  QT_ACCESSIBILITY=1
  QT_AUTO_SCREEN_SCALE_FACTOR=0
  QT_SCALE_FACTOR=1
  SESSION_MANAGER=local/debtest:@/tmp/.ICE-unix/999,unix/debtest:/tmp/.ICE-unix/999
  SHELL=/bin/bash
  SHLVL=2
  SSH_AGENT_PID=1136
  SSH_AUTH_SOCK=/run/user/1000/keyring/ssh
  TERM=dumb
  TERMCAP=
  USER=kupfer
  XAUTHORITY=/home/kupfer/.Xauthority
  XDG_CURRENT_DESKTOP=MATE
  XDG_DATA_DIRS=/usr/share/mate:/usr/local/share/:/usr/share/
  XDG_GREETER_DATA_DIR=/var/lib/lightdm/data/kupfer
  XDG_RUNTIME_DIR=/run/user/1000
  XDG_SEAT=seat0
  XDG_SEAT_PATH=/org/freedesktop/DisplayManager/Seat0
  XDG_SESSION_CLASS=user
  XDG_SESSION_DESKTOP=mate
  XDG_SESSION_ID=3
  XDG_SESSION_PATH=/org/freedesktop/DisplayManager/Session0
  XDG_SESSION_TYPE=x11
  XDG_VTNR=7
  A__z="*MAILCHECK="*SHLVL
  debtest$ 

I tried unsetting IGNOREEOF; that didn't make a difference.  And
changing SHELL to /usr/bin/ksh didn't make a difference.  Maybe there's
something about EDITOR, TERM, or TERMCAP?

mike