can't do `backward-word' at end of line with certain timestamp config

I can't do `backward-word' with point at the end of an
inputted ERC line with this timestamp config and timestamps
disabled - nothing happens, the cursor doesn't move.

(setq erc-timestamp-format      "[%s]")
(setq erc-timestamp-format-left "[%F]\n")

(let ((insert-fun #'erc-insert-timestamp-right))
  (setq erc-insert-timestamp-function      insert-fun)
  (setq erc-insert-away-timestamp-function insert-fun) )

With this it works:

(setq erc-timestamp-format      nil)
(setq erc-timestamp-format-left nil)

(let ((insert-fun #'ignore))
  (setq erc-insert-timestamp-function      insert-fun)
  (setq erc-insert-away-timestamp-function insert-fun) )

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