Hey, coming back to this for a moment - now that we're doing LQA, the exact scope of the problem is pretty clear. The issue is, if breakText is false, STM tries to add a line break before the 「 or [ chars.
You can try it yourself with this text:
スライムとして生活すれば、それは本当に「自分」なのか？<d=0.5sec> 我の自我は「身体」と「心」のどっちでござる？<d=0.25sec> もはや違いはあるじゃろう？
Below is with breakText = false:
And this is with breakText = false:
The zero-width space didn't work, but using your idea about the pre-parse event, I decided to make a script to swap the breakText setting if the upcoming string contains one of those bracket chars, and then switch back to default after.
I'm guessing this all has something to do with the block of code at line 3596 in STM, but it looks too finnicky to edit myself.
I might have mentioned in some other thread, but I basically can't risk updating STM at this point, because any changes might break things unexpectedly in other parts of the game. So I'll probably stick to this pre-parse solution. But, the information is here, anyway. Right now, the asset isn't really usable in Japanese without this workaround, because setting breakText to false all the time isn't the best solution (it could cause too many line breaks if a sentence has several commas etc).