If you use separate tags next to each other, with a space in between, Xenforo seems to try to help by combining them into one tag.
For instance: one two becomes one two.
It seems to be an issue with the rich text editor, even if entering the bbcode directly. Once I publish or switch to BBCode view, the tags are combined. If entered directly in BB Code Editor, it works fine.
I've changed this in 1.3. We have a "tag simplification" process that reduces the tags to what should be an equivalent appearance but with fewer tags.
Unfortunately, we can't completely do this with general spacing, so I've adjusted it to only work across lines (which is what it was meant to handle, as the WYSIWYG editor would generate more tags than strictly necessary for each line.)