Okay I found my problem by reloading an old save. Apparently if you already spoke to Annie and unlocked the kids, the Forest would already show up as available. Going to the Forest without speaking to Annie again would somehow skip the Kana discovery scene entirely.
Viewing post in PUBLIC RELEASE - v0.20.2 - KANA COME HOME! comments
Actually, the only way to miss Kana is if she's either love level 3 or lower, or you chose the "That little idiot" dialogue option when talking to Annie. Everything else defaults to you finding her.
There is an edge case, however, where you could ignore Annie completely, go to the forest, rescue Kana, come back, then talk to Annie and have her speak on how Kana has vanished, so I've fixed that bug :)