What’s your completion framework?
Ido-mode? Or no completion framework?
If either of them, I believe that’s the standard behaviour (not bizarre from Emacs’ perspective).
You might like to try evaluating this:
(define-key minibuffer-local-completion-map (kbd "SPC") 'self-insert-command)
Source: emacs - How to enter a space in the minibuffer instead of completing a word? - Stack Overflow
It works on my end for Icomplete-mode, Ido-mode, and Emacs stock completion (no completion framework).
Searching this forum for “completion space” can also get you this: Using org-roam with ido, - #8 by nobiot