Is the true altitude always lower than the indicated altitude as long as a correct local altimeter (QNH) is set on the altimeter?
No, the true altitude can also be higher than the indicated altitude, if the temperature is higher than expected from the International Standard Atmosphere (ISA). This case is usually not as important as the opposite case because flying at higher true altitude does not create a risk due to terrain clearance.

In a standard atmosphere, the indicated altitude
reflects the true altitude above the mean sea level
(MSL) and therefore provides a reliable indication of
terrain clearance.
Whenever, the temperature deviates significantly
from the standard temperature, the indicated altitude
correspondingly deviates from the true altitude, as
follows:

Extreme high temperature: the true altitude is higher than the indicated
altitude,
Extreme low temperature: the true altitude is lower than the indicated
altitude, thus creating a lower than anticipated
terrain separation and a potential obstacle clearance hazard.



(Airbus - Approach-and-Landing Briefing Note - 3.1 - Altimeter Setting - Use of Radio Altimeter)