Selecting vertical text is offset by one character

[Pikrass]

On zathura 0.5.8 from the Debian repo

I've got books in Japanese, written top-to-bottom. They all display this same behavior: selecting a character actually copies the following character (the one below). To copy the first character of a line I have to select a "shadow" character above it.

Here's a screenshot of what it looks like when I select the whole rightmost line on this page:

This is not purely a visual bug, I actually had to click and drag from the top of the green area. The copied text includes all the characters on the line (including the final を).
As a side note, the selection area is also a bit offset to the right.