The forehead temperature reading can be affected by profuse sweating or if an oxygen mask is present or if the subject is elderly.
In case of sweating, dry the forehead is not enough, as the low readings do not depend on the sweat drops, but on the dissipation of heat which occurs when sweating.
The oxygen masks are not hermetic and so the oxygen can make the forehead cold: the temperature reading would not be correct.
A low vascularization of the forehead may occur in elderly people. In all the above cases, the temperature detected on the forehead will be lower than the real body temperature.