The sandstorm event added by DynamicSurroundings and which is supposed to happen in all desert type biomes when it rains, but it doesn't happen in some BOP biomes, surely because they are badly registered as DESERT. I noticed it didn't work on Outback and Oasis, but the list could be longer.

Those biomes are not intended to be registered as deserts though...

https://github.com/Glitchfiend/BiomesOPlenty/blob/BOP-1.11.x-6.0.x/src/main/java/biomesoplenty/common/init/ModBiomes.java#L486

https://github.com/Glitchfiend/BiomesOPlenty/blob/BOP-1.11.x-6.0.x/src/main/java/biomesoplenty/common/init/ModBiomes.java#L510

Note the definition of a desert (which doesn't quite fit either biome):

A waterless, desolate area of land with little or no vegetation, typically one covered with sand.

And furthermore, the DESERT type has been removed:

https://github.com/MinecraftForge/MinecraftForge/blob/f08f3c11053d414b57d03192dd72fcbfaef100f7/src/main/java/net/minecraftforge/common/BiomeDictionary.java#L36

Strangely it works with other desert like biomes like xeric shrubland and lush desert. The problem maybe comes from dynamic surroundings.

Definitely a problem with Dynamic Surroundings.

https://github.com/Abastro/BetterRain/blob/master/src/main/java/org/blockartistry/mod/DynSurround/client/storm/StormRenderer.java#L149

Here he checks whether a given biome has dust set. But as far as I can tell, there is absolutely nowhere that this value has been set to begin with...

https://github.com/Abastro/BetterRain/blob/master/src/main/java/org/blockartistry/mod/DynSurround/client/WeatherUtils.java

https://github.com/Abastro/BetterRain/blob/master/src/main/java/org/blockartistry/mod/DynSurround/data/BiomeRegistry.java#L75

I'll leave it up to the developers of Dynamic Surroundings to handle this.