Are these direct readings from the sensor or are you using filtering?
The sensor will reset to 88C (ca. 190F) on power up, and this can happen when doing a long conversion and you lose power. The sensor appears on the bus, since it’s able to use parasitic power, but as soon as you try to read a temperature it will fall off the bus.
Since the onewire protocol includes crc checks, a high reading seems unlikely from a firmware problem. (I don’t know the dallas library - so not 100% sure it has crc checks implemented.)