i am ot sure "sacrifice at the altar" infers a motion toward, to the altar. So if the priest is already at the altar, ready to sacrifice, translating by "in ara sacrificare" should be correct, shouldn't it?
I agree with you. There's not a verb of movement here. I think it should be "in ara".
Just made this "mistake". Thanks for asking this.
Just reported: "Sacerdos in ara sacrificare solet" should be accepted
It is not presently, but i will report it too.