hey there, what happened is this plant's pot probably didn't dry as quickly as the others or it dried quicker briefly causing a pH fluctuation , that's why you can see all sorts of symptoms from things that shouldnt be there, just keep an eye on this one's micro climate and position in the garden, nothing you can do now if the problem has stopped spreading. It will get slightly worse then be totally fine. Alternatively water with humic acid which stabilises your soil and feed pH to 6.5 , hope this helps ! đ