Update, 9 a.m. Pacific on Sunday, August 30: The game appears to be back on Apple’s App Store. Toto told VentureBeat he estimates the losses for Mixi to stand at about $600,000.

Apple has pulled Japan’s top-selling mobile game from its App Store in Japan, according to local media reports Saturday (link in Japanese).

Developer Mixi claimed earlier this month that its blockbuster title Monster Strike was making $4.2 million every day ($378 million from April 1 through June 30), while spending very little on marketing. That translates to about $175,000 per hour.

The tweet below in Japanese (as gaming analyst Serkan Toto spotted) claims to show Monster Strike’s absence from the top five grossing games in Japan’s App Store as of 6:00 PM local time Saturday:

Mixi has since put up a blog post (link in Japanese), which roughly translates to: “The use of serial codes are being stopped with immediate effect, making many people believe that this is the main reason Apple pulled the plug.”

While not confirmed, it seems that Apple may have decided Mixi was violating clause 11.3 of the App Store review guidelines on purchasing and currencies: “Apps using IAP to purchase physical goods or goods and services used outside of the App will be rejected.”

It’s possible that Apple also has qualms with Mixi’s adherence to clause 11.2: “Apps utilizing a system other than the In-App Purchase API (IAP) to purchase content, functionality, or services in an App will be rejected.”

Anton Soeharyo, the CEO and founder at Jakarta-based mobile game studio Touchten Games (which recently took funding from Japan’s Gree), offered VentureBeat the following comments:

The real reasons are unknown, except to Apple and Mixi. For obvious reasons, Mixi does not want to reveal the ‘real reason’. But according to the way it announced the inability for users to use their ‘serial code’ system in the blog posting — and according to other speculation (link in Japanese) — I think one of the reason is that by giving ‘referral codes’ that can unlock certain in-app purchasable items, it was bypassing the IAP system highlighted in clause 11.2. Apple has repeatedly warned Monster Strike to delete this feature, but they didn’t. So that is why they pulled the plug, it seems.

Soeharyo said he takes a neutral stance, but admitted the news has shocked the gaming community, and acts as a reminder of Apple’s ultimate power in these matters.

“I am just constantly being reminded of how fragile and unpredictable this business is,” he said.

The New York Times reported in December how Monster Strike has given the once-doomed Mixi a chance at “self-reinvention”. In that report, Serkan Toto was quoted as saying, “They were on their knees when Monster Strike came out.”

VentureBeat has reached out to Apple and Mixi for comment, and we’ll update you if we hear back.