The short answer is that the appearance of cracked darker floor is random after every server save, therefore a daily check is necessary in order to know if it's there and if so, to collect the key 7909 downstairs (use squeezing gear of girlpower on the crack). The key 7909 itself is used to open a door in the mines that leads to one of Ada's chests, as mentioned in the Tibiawiki link you posted in the question.
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For a longer background story, this is a bit related to the thirteen Ada's chests. If you're familiarized with the Roshamuul bottles (achievement "Nevermending Story"), these chests work similar to the bottles in the sense that every server save, there's a chance one will spawn.
In Roshamuul, a bottle can spawn in one of the 11 locations and you have to "fish" a message out. In Ada, there are 13 chests spread out across the whole isle and every server save, there's a chance one of them (or none) will be a "mimic chest", where a biting animation will appear when you click on the brass-shod chest.
There's a belief that one of the secret achievements from 2025 Winter Update might be clicking 13 different mimic chests, so there are a handful of people clicking all these chests daily, including my server, to try to get these "mimics".
So far, it's been noticed that when the chest where this dark floor is located is a mimic, the cracked dark floor is always there. The only thing I'm not able to confirm if it "spawns" at server save as I only play at night time. But what I can say is every mimic that's been reported on my server, I've been able to click it even 12h later.
