The soil around the sensor should be tight, otherwise the moisture reading will be too low and it will be difficult to train the sensor to understand the soil. Could you please check if you insert the sensor tightly?
The low moisture might also be caused by the rising temperature and more sunlight changing the environment. The system is aware of this change and in fact the low moisture will increase the watering. In your case, the change may be too fast and the self-learning mechanism has not responded too it timely. Once the sensor adapts to the weather it will report the report the proper moisture level.