You can't tell from the URL, but this link is to a preprint [0], and the bad boxes there are intentional, since we compile all the preprints in draft mode [1]. The final article will be published some time in August [2], and should be perfectly typeset.

[0]: https://tug.org/tug2026/program.html#:~:text=Clemens%C2%A0Lo...

[1]: https://tug.org/tug2026/cfp.html#preprints

[2]: https://tug.org/TUGboat/#:~:text=vol%2E47%2C%20no%2E2%20%28T...