Yesterday I had a short relay run and the watch displayed a 234 max heart rate, which is quite high, even for me
Wrist based measurements are not very trustworthy, so I would guess it is most likely wrong in this case. Is it by any chance closely linked with the running cadence you had on your stage?
Is that wrist based, or with a chest strap? A friend of mine reported 225 on his stage, which is also unusually high for him.
Wrist