So as official artist for the game, Ooyari Ashito is allowed to make lewd fanbook of, I assume other ships other than the one(s) drawn by them? Is this actually more common than I thought?

Gotta draw in the crowd (and money) somehow.
If the fanon won't do it, you gotta do it yourself.

A knockoff of a knockoff. Neat.

that or Yostar actually saw Oyari's Unicorn doujin and said yeah... MORE!!!... but what do I know... Yostar has already acquired Hentai artists to make ship girls...

Frawnkenstein said:

So as official artist for the game, Ooyari Ashito is allowed to make lewd fanbook of, I assume other ships other than the one(s) drawn by them? Is this actually more common than I thought?

He already made an AL doujin before he was hired to make an official one. I do think he is the first artist to actually made a doujin despite he is now working in said game.

Shiratama also had a Unicorn Doujin she's the artist behind Fumizuki...
https://twitter.com/shiratamacaron/status/949589327469662208