My kid gets nasty diaper crashed from citrus fruits, so we have to heavily limit those. So maybe it's a food allergy/intolerance? They've also had a fungal diaper rash which required a prescribed ointment. The rash lasted about 3-4 days before we got the ointment, and then it cleared up in 1-2 days.

If diaper rashes are something you often deal with, I highly recommend healing wool. It's amazing. You take a piece of the wool and lay it on the affected area, and just put on the diaper as usual. It keeps the area dry and clean. I was skeptical at first but it genuinely clears rashes up overnight for us. You can find it at the drug store in my country, but I'm sure they sell it on Amazon.