Although beans aren't heavy feeders, corn will take up the phosphorus. However, that does seem like a big dip if your remaining fields aren't showing similar numbers and they were treated the same. How was your plant vigor and yield in 2020? Could this one test be off? It's important to take the sample from the right depth. You probably already knew that though, and if you took the sample, I don't doubt you did it correctly. Adding usable phosphorus to the soil is an easy fix at least. Usually it's erosion that leeches phosphorus, but you've already said that it didn't rain more than usual. I'm at a loss.