As others pointed out there are ways to trigger the sensors. I have >90% success stopping on middle of the dipole sensors. Just a single aluminum wheel is enough for many of them.

Also consider that some left turn signals may require more than one vehicle to trigger, however in those designs you just don't get a protected left. Or others you have to get to more than a moment before the thru light turns green to get a left signal otherwise you have to wait a 2nd cycle.

So yeah, if you can't trigger it, then proceed as if a stop sign. But also call authorities and report it as malfunctioning.

Al