Backspace echoed incorrectly with TERM=garbage

[bryanh]

Configuration Information [Automatically generated, do not change]:
Machine: x86_64
OS: linux-gnu
Compiler: gcc
Compilation CFLAGS: -g -O2 -fdebug-prefix-map=/build/bash-2bxm7h/bash-5.0=. 
-fstack-protector-strong -Wformat -Werror=format-security -Wall 
-Wno-parentheses -Wno-format-security
uname output: Linux rhino.giraffe-data.com 4.19.0-9-amd64 #1 SMP Debian 
4.19.118-2+deb10u1 (2020-06-07) x86_64 GNU/Linux
Machine Type: x86_64-pc-linux-gnu

Bash Version: 5.0
Patch Level: 3
Release Status: release

Description:

  With TERM environment variable set to an undefined terminal type, Bash
  echoes a backspace as a space; I expect it to echo as a backspace (ctl-h).
  It edits the line properly; it just isn't displayed correctly.
  
  Other line editing functions have the same problem.
  
  Setting TERM from the Bash prompt (to anything) puts things back to normal.
  
  It worked (Backspace echoes as a ctl-H backspace character) in an earlier
  version of Bash (don't know which one), but not in 5.0.3(1) from Debian or
  5.1 from ftp.gnu.org.

Repeat-By:

  $ TERM=nosuchterm bash --norc

  At the prompt, type the 7 keystrokes

    l s q q BS BS CR

  It echoes

    l s q q SP SP CR LF

  and performs an 'ls' command, showing that the line was properly edited.