Custom fonts stopped working on Paperwhite 10th gen (PW4) after removing the jailbrea

[icyborealskyes]

Hi all,
I have a Kindle Paperwhite 10th gen (PW4) on firmware 5.18.1.1.1. Custom fonts were working fine through the official fonts folder before I jailbroke it. After I removed the jailbreak, they stopped working, and I can't figure out why.
What I did to unjailbreak:
Ran the uninstall script from the guide on kindlemodding.org.
Did a factory reset.
Reinstalled the official firmware for my model (5.18.1.1.1, the same version I had before).
What I've tried since:
The fonts folder exists and contains only the README created by the firmware.
I copied static .ttf fonts (e.g. from Google Fonts) with all 4 variants (Regular, Bold, Italic, BoldItalic), then restarted the device. I also tried .otf files earlier.
Tested in sideloaded AZW3 books (converted with Calibre), checking Aa → Font at the end of the list.
Embedding the font with Calibre and selecting Publisher Font also doesn't work.
The fonts never appear in the Aa menu. The firmware version is the same as before, so I don't think it's a firmware change.
Has anyone seen this? Could the jailbreak or uninstall leave something behind that affects custom fonts, even after a factory reset? Any file or setting I should check? Thanks!