The Sith conduct experiments with the Force and they have changed the Force many times (for example, the Grand Experiment). Provided that the Jedi have researched both the Force and the Sith, it's safe to assume that they know this fact.
If the Force isn't clean and immutable, why do Jedi trust the Force to make decisions?
Your question actually assumes a certain philosophical perspective of Jedi called The Living Force. Jedi who follow this theory tend to be more willing to make snap decisions in the heat of the moment, because they believe in being focused on the present. As you put it, they "trust the force to make decisions".
There are many Jedi who do not make snap decisions and instead choose to meditate on the topic at hand in order to come to a conclusion.
The true nature of the force is unknown, and it is the subject of canonical debate amongst many interesting characters. Even the Jedi have two major competing schools of thought. The Living Force and The Unifying Force. The statement "changed the force" doesn't hold much weight here because we don't know if they have changed the force. We only know that it was their intention because their particular religious interpretation of the force tells them they can.
Which brings me onto my final point about the Jedi. Your question is why do they trust the force, even in the face of the potential deception. It's true; from an objective point of view it might seem somewhat risky, but they are a religious order. They exist to study the mysteries of the force, and circumstances often arise that forces them into a violent conflict, in which they are at risk. They have faith that the force is their ally and the light side of the force is the true path. There is no reason why; it's just their particular doctrine.
The correct answer is very simple - because it works. They trust the Force, and it guides them to correct decisions, in multitude of situations.
Digging deeper: as to WHY that's the case if the Force is mutable:
Yes, the Force isn't immutable as an absolute thing, and Sith can change it in some rare cases (E.g. Plagueis or Sidious).
But that's NOT a common occurrence, and doesn't happen for random things.
As such, an individual Jedi CAN rely on the Force for a majority of decisions.
As a good out-of-Universe analogy, consider this: Newton's laws of motion are wrong according to Einstein's both Special and General Relativity.
BUT, for a majority of mechanical calculations that most humans do and rely on, relativity effects are so insignificant, that they CAN rely on Newton's laws, without worrying that they are actually NOT 100% correct if you account for relativistic effects.
Also, when it matters, they DO know when they cannot rely on the Force, such as Yoda indicating that they can't predict the future during the events of the prequels, precisely because the Sith clouded the Force.
The Jedi may detect the "disturbance in the force" caused by changing it. All things are interconnected and a theoretically all things, everywhere, can be sensed. The force as a whole will always be neutral, always in conflict between light and dark. Changes made by users are irrelevant in the grand scheme of things. The Jedi know the force as it truly is and will always be in harmony with the will of the force.
