Thank you for this question, Yuya Takizaki.
Yes , empty subtitles must be deleted before publishing, because that's how the software works.
Moreover, if you merge the content of two subtitles, you'll probably need to lengthen the resulting subtitle.
You can keep the original and translated subtitles at the same height by clicking on the small lock
Yuya Takizaki
How do you erase the grey square behind subtitle texts?
I merged two subtitles and then, I insert blank subtitle to simplify to campare original with translated easily.
is to delete blank subtitle right before publishing the only way?