It seems to be related to the individual 7135 chip used. Some do it, some don’t. Or at least, some do it enough to be noticeable, while others have a much much more subtle effect.
In any case, I think I found a way to reduce it further, and sent an update to Fireflies. Hopefully it will be used in lights which haven’t been assembled yet.
It is also possible to make moon lower and smoother by increasing the MCU clock speed, but that makes it use 3X as much power to produce 1/3rd as much light, and it also makes the moon mode a lot more sensitive to small changes in hardware and battery voltage.