Alcor via General discussion about ERC <emacs-erc@gnu.org> writes:

> 1. Start from a clean Emacs installation (-Q or default config).
> 2. Install the latest stable version of Erc via C-u M-x package-install
> erc RET (At the time of writing, this is Erc 5.6).
> 3. Connect to Libera.Chat under any nick.
> 4. Once connected, type /QUERY fsbot and hit RET.
> 5. Interact with the bot in any way that forces a reply (e.g. type
> ,greet RET).
> 6. Wait for the bot to reply.
> 7. Trigger the undo function via M-x undo or C-x u.
> 8. Observe as the undo affects the reply text inserted by the bot.
>
> This is a regression from Erc 5.5, where `undo' only affected the text
> typed by the user within the input prompt.

Thanks for reporting this.

Hoping this patch will address the issue.