I believe the term you're looking for is reciprocity failure[1] (or schwarzschild effect), though apparently it doesn't happen with digital sensors - only film. Noise becomes an issue in similar situations, but it's not affected by the number of photons hitting the sensor.

Correction: Shot noise[2] is related to the number of photons hitting the sensor.

[1] https://en.wikipedia.org/wiki/Reciprocity_(photography)#Reci...

[2] https://en.wikipedia.org/wiki/Shot_noise