I selected your formatted text with the mouse and hit the grey Quote button to stick it in this reply. As you can see, spaces now appear where there were none before. I got identical results using Copy and Paste. I’m calling this definitely buggy behaviour, because the text was already rendered a certain way, so why should it change when it’s copied?
Seems doubly flaky if it inconsistently failed to put in spaces when you quoted your own post.
A wild guess is that the funny business happens when text on your screen that you select and paste is run by Discourse through an HTML parser.
Test: here is a sentence copied/pasted from Wikipedia:
Various events in Fitzgerald’s youth are reflected throughout The Great Gatsby .
Note the space that wasn’t there originally. I’ll leave it up to you all to file the bug report…
foobar -> foo bar so it’s not just before punctuation.