Minimal configuration:
(use-package erc
  :config
  (setopt erc-modules '(scrolltobottom))
  (erc-scrolltobottom-mode))

1. Use above config to ensure scrolltobottom module is loaded
2. Start erc with a connection to libera (M-x erc-tls)
3. C-x 1 on the erc buffer window to make it the sole window
4. Once the Libera buffer is loaded, this should be a sufficient number
of lines to trigger scroll to bottom.
5. On some resolutions or window size this may be enough to see that the
input line (ERC>) is not at the bottom of the window, but in the
middle. If not, proceed to step 6.
6. Invoke (global-text-scale-adjust) using C-x C-M-= and continue to
press the "=" key until you can notice that the input line jumps to the
center of the window.

Attached, find a video of the issue.