Unsure if this is intentional, but it appears that Oak Signs & Oak Hanging Signs drop Walnut variants of themselves. After testing, none of the other Vanilla signs have this issue, and neither do any other Oak-based blocks.

The problematic files appear to be located at generated/oak_sign and generated/oak_hanging_sign, where the edited drops explicitly show the Walnut variants. The datagen files don't seem to have anything that'd cause this issue; might just be residual generated files.

(Love the mod, by the way - one of the most polished farming mods I've played in a while)