For pH meters that are specifically designed to be inserted into the soil, this is a question that frequently gets brought up by customers on Amazon. The only issue is from one user to the next, the answer tends to differ. For example, one customer claims that when they left the probe in the soil, the sensor tip deteriorated over time. But, then you have others who report that they have left them inside with little to no issue.
However, in order for you to get a correct pH reading each time, you will want to remove it. To ensure the most accurate reading possible, you should wipe the dirt off of the probes with a dry cloth before each use. So while leaving it inside the soil may not damage it, there really is not a point as this will not provide you with an accurate reading.