Yeah I noticed this last night while playing a match with my brother with me picking Olimar, and him Piranha Plant. However, it's important to note that in Smash Bros, White Pikmin use Darkness as their element instead of Poison. Currently, Piranha Plant's Poison Breath deals Poison damage, which as an element just got introduced in SSBU. But it does use the the Darkness effect as well in a strange way. Also I don't think White Pikmin were ever implimented to be immune to Darkness, unlike how Red, Blue, and Yellow Pikmin are immune to Fire, Water/Ice, and Electricity. So I wonder if in another future update we'll see White Pikmin gain a buff in being immune to Darkness/Poison, which would really effect immunity to certain attacks by Ganondorf, Mewtwo, and Robin. Only time will tell if this oversight gets fixed.