It is a temporary bug and the team has applied a fix to make it automatically move to the mainboard.
1. Collect the seed chest (any 'Save the plant' event items') to have it moved to the flower board from the queue;
2. Force-close the app;
3. Reenter to the game.

If you have space on your mainboard, the seed box will go to the mainboard. If there is no free space, it will go to the queue in between the 'Inventory' and 'Shop' icons.