Many dyno powered lights should be mounted right side up. Not sure why but many of the dynopowered lights have a vent hole on the bottom that could collect rain if you mount it upside down. So, inspect your light closely or maybe do a pertinent google search to see if it has a hole that is supposed to be on the bottom, not top.

I have not noticed vent or drain holes on battery powered lights, but if a light had a lot of waterproofing seals it might also have a drain hole to help equalize pressure.

I recently bought a dynopowered taillight and was a bit dismayed when I saw it had a vent hole on the bottom, if I mount it on a rack without a fender I am sure much of the tire spray will go upwards towards the light.