Why don't electronic thermometer "move back"?
The action of the digital clinical thermometer is designed to hold the peak reading so that it can be read at leisure by the user. This was actually a feature of the classic mercury thermometer and is replicated by the digital thermometer firmware.


To use the thermometer, the bulb is placed in the location where the temperature is to be measured and left long enough to be certain to reach thermal equilibrium—typically three minutes. Maximum-reading is achieved by means of a constriction in the neck close to the bulb. As the temperature of the bulb rises, the liquid expands up the tube through the constriction.
When the temperature falls, the column of liquid breaks at the constriction and cannot return to the bulb, thus remaining stationary in the tube. After reading the value, the thermometer must be reset by repeatedly swinging it sharply to shake the liquid back through the constriction. Source: Medical thermometer. Emphasis mine.*

Note that the mercury thermometer is evacuated (a vacuum) so there is no air pressure to push the mercury back into the bulb. When the bulb cooled the mercury column broke at the constriction and there was vacuum at both sides of the thin column of mercury on the scaled portion of the instrument. A centrifugal shake (bulb outermost) returned the mercury to the bulb.