Do "recipe give @p *" to get all the recipes, then right-click a crafting table. Open the recipe book.

Most searches will find any items whose full text of the tooltip matches whatever you type in the search box. Notably, searches like "att" or "hand" show all weapons and tools, since they list "Attack Damage" and "when in main hand" as part of the tooltip. This is fine and good.

EXPECTED

The subset of recipes that appears for a search <foo> is all of the recipes whose output item has <foo> as a substring of its tooltip text.

ACTUAL

Try a search like "6". Some of the search results like "Iron Chestplate...+6 Armor" or "Diamond Leggings... +6 Armor" make sense. But "Wooden Axe" also shows up (0.8 attack speed, 7 attack damage) despite containing no "6". Other numerals have similar non-sensical results.

Note that if and when custom recipes can provide NBT outputs (e.g. name/lore), I expect the recipe book search to utilize the custom name and lore.