Not sure if it's just me, but I've been having issues after quoting people - Windows, Chrome both at home and at work on PC's. I can quote something and when responding, cannot insert a space. I resolve it by going up to the text I have quoted, highlighting a character or two, copy, then paste into my new text and everything works fine. I doubt it is something with the site, but it has been going on for a month or two. I just tried it in a Microsoft Edge browser and it works fine.
Same problem here for past few weeks.
Thought it might be a Chrome issue.
Changing font size didn't help.
Not doing it on this message, but, it's the first one today that hasn't exhibited exactly what you describe.