No. There is no bug here. This is the behavior by design, like
it or not.
Do I personally think that `SPC' should generally be self-inserting
during completion? You bet I do. And so should `?'. And so should
`C-j' (newline). (And this is the case in Icicle mode, for instance.)
But that is not the opinion of Emacs Dev. It took decades to finally
get `SPC' to be self-inserting for file-name completion (see variable
`minibuffer-local-filename-completion-map', in the same Elisp manual
node). Patience. ;-)