It’s because the foundation is only 9x blocks and not connected to the other foundation from what I can see from your screenshot. That means this foundation and anything on it will disappear within xx hours due to the decay system.
I know, it sucks. But it’s not a bug. This is intentionally, to prevent players to claim land.
The same thing happened to me, see my post in this forum called: “decay system is OP”.
Solution: Do not build small, separate base sections.