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Amazon's KF8 (azw3) format supports vertical text, but the old MOBI-7 format does not.
The first attachment (無職転生 - 蛇足編 - 番外編.mobi) is a Master MOBI which contains both MOBI-7 and KF8 in a single file. This type of file is produced by Amazon's Kindle Previewer or kindlegen. The reason that it works is that devices capable of displaying KF8 make use of that part of the file and ignore the MOBI-7 part.
The second attachment (After Comverting.mobi) is plain MOBI-7 format, without KF8. That is why it is so much smaller and why it does not support vertical text.
As Kovid stated, you need to convert to a format that supports vertical text, such as EPUB or KF8 (azw3) for vertical text to work properly.
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