Readline 8.1 is required for bash-5.1 to work properly.
From bash-5.1 release message[1]:
> Bash can be linked against an already-installed Readline library
> rather than the private version in lib/readline if desired. Only
> readline-8.1 and later versions are able to provide all of the symbols
> that bash-5.1 requires; earlier versions of the Readline library will
> not work correctly.
[1] https://lists.gnu.org/archive/html/info-gnu/2020-12/msg00003.html
090b1804b7