Okay yes, I'm now messing with this myself in Unity (sorry, I really thought it was just a font texture thing...) and I can now see it has something to do with the cutoff point... dragging the bounds causes the "de" right before the font change to disappear with certain line widths... going to be looking for the fix now, it may be some of the code I wrote that specifically handles linebreaks for japanese characters.
Replying immediately, because I think that you may get the result you want right away by using the monospace numerals from japanese without any problems (０１２３...) but I want to solve this for the way you're doing it, too. It looks like I haven't included these monospace numbers in the japanese linebreak rules either, so I'll make sure to add those, too