fan speed regulation causes other loads flickkering
Triacs cause harmonic distortion of the current used by the fan. If that current is a high proportion of the current used in the circuit and the circuit is near capacity, the current distortion can cause voltage distortion. The voltage distortion can cause flickering of the lighting. The difference in the two location is likely that the flickering location has lower current capacity and/or longer wiring distances compared to locations where there is no flickering.