Is there any reason why "Forte advenimus ad arborem" shouldn't be accepted? I hope not, because I reported it (30/09/19).
There's no reason. You have Adv, V, O. So it's the same than their Adv, O, V.
(For the purists, in reality "at the tree" is not an object in English, but it doesn't change anything here.)
Their version is simply more common.
Just report and wait.