I have seen a release of water sprayed onto a high-pressure GH2 vent. Is that a normal behavior?

Spraying water onto a vent stack, either for gaseous hydrogen or liquid hydrogen, is not recommended. While this is prohibited within the code for liquid hydrogen due to the much greater hazard of plugging the vent system, it also presents hazards for gaseous vents as well. The water can enter the vent system and plug due to ambient conditions. In addition, if the water was sufficient to extinguish the fire before the hydrogen flow is stopped, then a flammable or explosive cloud may form which can reignite unexpectedly leading to a greater hazard. 

Water is only recommended to cool equipment adjacent to a hydrogen fire.