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Possible explanation:
The floor is equipped with floor heating. There are pipes inside the screed of the floor that are connected to the heating system. I assume that the floor has been constructed recently, and that the picture shows the result of the first usage of the floor heating, while the floor was installed too hastily.
What could have happened here is that the screed floor with the floor heating pipes has not been let dry enough before it was covered with floor tiles. When installiing a screed floor there is a lot of moisture from the screed present. It needs to evaporate first. It usually is done by running the floor heating for several weeks without having the floor tiles installed yet. This allows the remaining moisture to evaporate through the screed. The tiles usually are installed when the screed has been dried enough.
When the floor is covered with tiles without letting it dry properly first, the tiles seal the floor and remaining moisture cannot evaporate. When the floor heating is in operation, the trapped water vaporises and can't escape the floor. Due to the pressure that is being created, the floor bursts open violently. Compare it to water that is being heated in a pressure cooker and it can't escape.
Hm. My heated floors run at about 30°C on a very cold day. Do you think this could actually happen at that temperature?
(But yes to the drying part. We had the heat pump for the floors and multiple air dryers on full blast during our first winter, concrete floors, skreet and stucco in a house contain several m³ of water and not nearly all of that gets chemically bound)
My heated floors run at about 30°C on a very cold day.
That seens to be normal on a cold day. Your floor heating should be fine. Once the moisture has had a chance to be evaporated, any operating temperature (within its range, of course) for a floor heating system should be ok. I suspect that the problem in the picture is that the moisture has been trapped inside the floor, because the floor tiles sealed it up.
When moisture cannot escape, it goes for the weakest point - in that case the floor surface, because it can't go downwards (water vapor always rises up) and it can't go sideways because of the walls of that room (the screed floor is between the walls). The temperature the floor heating could have been operated with didn't need to be that high, just hight enough to make the remaining water start to change its aggregate state. Water vapor takes up more space than liquid.
The ammount of water that is brought into a buildung with concrete, screed, plaster, etc. (basically all material that has been a pile of dirt once) is often underestimated.
Water vapor absolutely can go down when under pressure. It goes in all directions in accordance with the pressure placed on it until it equalizes. In atmospheric conditions it tends to be more buoyant than air. But once it’s sealed it does wherever it faces the least resistance. You can put release valves facing down, it’s just also really stupid to do that because once released steam will just go up