When I look at the Crafter Manager, I can see the extreme patterns among the normal patterns. However, as soon as I want to search for one via the search box, no results are found for the extreme patterns. It's like the search doesn't know they exist. It doesn't matter whether those patterns are in a normal crafter or in a multi-block crafter.

ModPack: Dungeons, Dragons, and Space Shuttles v6.5

Refined Avaritia version: 1.12.2-2.6
Refined Storage version: 1.6.15
Reborn Storage version: 3.3.4.85

https://github.com/raoulvdberge/refinedstorage/blob/mc1.12/src/main/java/com/raoulvdberge/refinedstorage/container/ContainerCrafterManager.java#L177

The issue stems from here. The crafter manager uses RS's pattern implementation instead of the API interface.

Ah, okay. Do I need to inform the dev of RS about this myself?

Maybe, but I doubt they'll be fixing any bugs of this type for 1.12 anymore. They had me PR a fix for their crafting system not supporting more than 9 items to make my mod work.