βραχύς is of course oxytone; but with the prefix πρό, the accent needs to shift back to proparoxytone. So I suspect προβραχύς is simply an error.
πρόβραχυς appears only in Diomedes Grammaticus (4th cent.), who wrote in Latin with a few Greek words interspersed. I couldn't access Keil's 1857 edition (Grammatici Latini, vol. 1).