Prettier does not support Astro files natively, so it's the Astro plugin's fault in this case.

(I maintain said plugin, so, well, it's my fault)

Ha! Well, I’m happy to help troubleshoot if you’d like?

Wouldn't it be better to show the intended font at first page (using font-display: swap or fallback instead of optional) instead of a potentially uglier fallback font until second page?

With swap (or fallback) the page is visible early using a fallback font but you still end up with your "goal font" without having to reload or visit another page