This does not affect typical worlds that are generated as usual, only those that were created as a buffet world set to a dry biome before updating to 1.18. If a cave biome is somehow implemented underneath this "dry biome variant only" world after converting to 1.18, then lightning will refuse to summon, even if rain is reaching the lightning rod. Do also take note that in normal worlds, lightning will still strike lightning rods in dry biomes, but refuse to do so in buffet worlds set to dry biomes.