markdown-ts-mode not recognized after updating built-in packages

Hi,

I noticed recently that after updating the built-in packages (emacs
31.0.90), opening a markdown file with the built-in markdown-ts-mode
doesn't not enable the major mode. An echo area message appears
mentioning to uninstall the package since it is built-in since emacs 31.

This is not an issue with the corresponding tagged releases of the
built-in packages when installing emacs 31.0.90 from scratch. I also
have an `:ensure nil` keyword in my use-package macro for
markdown-ts-mode.

Is this behaviour expected? Thanks.

-- 
Kind regards,
Ashish