earthlover corners reserved for absent players - bug or real rule?

[Cos-]

I started my first game of Undergrove on BGA yesterday a two player game. After each of us placed our initial seedlings, and it was my first real turn, I chose reproduce and wanted to put my seedling on one of the two unoccupied corners of the central earthlover tile. But both of those corners were shown as black circles, rather than white, and clicking on them had no effect. I had to place the seedling somewhere else. As we've continued the game, those two spots remain as black circles and we can't put seedlings there. I don't remember a rule about this in the rulebook. Is this a bug?

[Jellby]

According to the tutorial: "Since this is a 2-player game, black circles were placed on the remaining two locations, blocking them."

According to the manual: "In a 2 or 3 player game, place Seedlings from the unused player colors in the un-picked corners of the center Mushroom. These corners are not available during the rest of the game."