At a loss of GPS signal situation, the app uses the tariff’s waiting mode setup in order to accrue the fare. When the signal returns, it calculates the distance between GPS loss and the new signal. It compares then the temporarily calculated value with the newly calculated value and takes the higher of both.
In cases where the tunnel is a straight line on a map, the calculated value will be correct.
In cases where the tunnel is not a straight line on the map, there might be a small deviation in the calculations.
On a more general note, if you need to drive a lot through tunnels, OBD is the safer choice.