From emacs -Q

  1. M-: (setopt erc-keep-place-indicator-follow t) RET
     M-x erc RET ... RET

  2. In the server buffer, do M-x erc-keep-place-indicator-mode RET

  3. Scroll "up" to hide the indicator but not so far that point moves
     beyond `erc-insert-marker'

  4. Switch away to another buffer and then back

  5. Notice the indicator fails to update

This happens because `keep-place-indicator' is a local module, and it
adds `erc--keep-place-indicator-on-window-buffer-change' to the hook
`window-buffer-change-functions' locally. But that function needs to run
in the buffer that's being switched to, which may not have the mode
enabled (and thus not have the hook member available). Subscribing to
the hook globally should fix the problem. There are also at least two
bugs affecting the integration between `keep-place-indicator' and
`keep-place'. The attached patch attempts to address these as well.