Definitely not the dimmer. I pulled it out of the circuit, put in a box and wired through the junction with a normal toggle switch and they do it exactly he same.
I don't think it's mains brown out, my HVAC system logs at ten minute intervals and the average voltage is stable. In addition, the driver is a 120->40V CC SMPS and not a linear driver like in cheap light bulbs, so slow changes in the RMS level should be compensated for by the current feedback.
I didn't hook up a photodiode, but by eye it looks like a few hertz periodic modulation of the drivers target current, like if someone will moving a dimmer switch from 80-100% moderately quickly. There's no shimmer or stroboscopic effect, just low frequency modulation.
Power company doing something after hours is plausible, as our trees. It only happens sometimes with no obvious patterns. I don't have a probe of the mains waveform, but we have long supply lines through a nearby forest. Short of buying mains probes, anything else I should look into?