The domain of the function is the set of real numbers, but the interval where the function displays unexpected behavior is x >= 100.

[CODE]:
```python
def f(x):
    if x < 100:
        # Some decreasing function
        return some_decreasing_function(x)
    else:
        # Some increasing function
        return some_increasing_function(x)
```

Note: The code above is just an approximation based on the observations and may not capture the exact behavior of the function f(x).