Turning on or off the candle (light) with the inline switch trips the GFI on the hair dryer - but never the wall mounted one. Push the reset button, and all's well.
I've measured some receptacle GFCI's tripping at 6mA and others at 4.2mA, which tend to nuisance trip more with motor loads and magnetic-ballasted lights. Inrush current is a common factor between such induction loads, and 1800W+ hair dryers.
Since, voltage drop increases with amperage, with high inrush current the GFCI may momentary detect more IČR losses across the (Hot with switch leg) than neutral without switch-leg, due to different impedance.
GFCI's may not always mix well (nuisance trip) with switch legs and high inrush currents. If the GFCI can't be replaced, try feeding the light (switch-leg wiring) from line side of GFCI.
Does anyone know if Residual Current Devices (RCD) and GFCI's trip at similar mA settings, or what the different standards are?