wp_trim_excerpt() calls wp_trim_words() which applies wp_strip_all_tags() to the excerpt. The <br> tag is removed and not replaced with anything.
Not sure if this is feasible, but having a space after a colon is grammatically correct, and adding it to the original content would not affect the HTML, e.g.:
<p>My new question is: <br>why words are not separated by a white space?</p>