Originally posted this as a bash bug but this behavior is implemented in readline itself so it's more fitting here.

When using vi mode, whenever a count is used before a command, readline replaces the prompt with (arg: n) where n is the count entered. This causes the entire line to shift, which is particularly jarring if a long prompt is used in a shell like bash, as is common. It should instead silently accept the count as it does any other input in command mode. This is how it behaves in other shells that implement vi mode including dash, zsh and ksh. There's also no mention of a prompt change in the POSIX spec regarding vi mode.

Replacing the prompt makes more sense in emacs mode to alert the user that a different mode has been entered. In vi mode, however, one is already in command mode when using a count so no further notification is needed.