Informative issue for people looking to report this.

This seems to be an issue with resource packs that modify fonts, not Patchouli.

Patchouli simply uses the unicode font that Minecraft itself uses.

To see if it's a problem with your resource pack, go to the "language" menu and enable "force unicode font". If everything in vanilla turns into boxes, then it's the resource pack's fault.

Example (using faithful 32x 1.15):