Stumped by this beauty on Level 17:
あのヤクザは薬物取引をしているそうだ。
I know I learned 取 and 引. Here they both take their kunyomi readings と and ひ respectively but the word is read とりひき. Does this often happen often where the hiragana parts aren’t written but you’re just supposed to know to include them??? 
Is there a name for this phenomenon?
How can you tell it’s not just an irregularly pronounced jukugo that you have to learn? Because of the Kun’yomi? I asked a question a couple of weeks ago about the use of Onyomi and Kunyomi, and I was cautioned that some of the more recent compounds still use Kun pronunciations. I wonder if this is something like that.
After a bit of looking around, this seems to also be connected to verb vs noun usage. The style guide says it’s acceptable to drop the okurigana for the noun form if the meaning of the word is clear.
So it’s less about a compound word (usually on’yomi) using kun’yomi, but of the okurigana being omitted.
Sorry, I couldn’t really find non-native sources.
It’s also written as 取り引き, but it’s often shortened to 取引. But it’s still pronounced the same way as if it hadn’t been shortened. This happens often with compounds with okurigana (the hiragana at the end). For example 入り口 is often shortened to 入口. They mean the same thing, it’s just there’s not much of a point writing that り in the middle since everyone understands it either way
受付 is another WaniKani word you might remember that fits this pattern.
立入禁止 is another common one! dropping the ち and り from 立ち入り.
I noticed this with 締め切り which can be shortened to 締切