I got the same letter for my bx25d. Different ref number. Kubota didn’t know why when I phoned. Issue was with over flow cap on cooling system. That was addressed years ago.
I had the same problem. Turned out there was a Little Rock ( approx 1/8 in in dia) between the linkage and the frame. Wouldn't let the peddle go all the way down. Couldn't see it. Couldn't feel it. Blew it out and all was fine.
I just got a set of stabilizers for my BX. Beautifully made. Mark made them and shipped them and there were no complications in the transaction. Way stronger and easier to use than the stock ones. Get them.