however numbering the bond 1 means there's ambiguity between it being between 1 and 2 and between 1 and 6
It might appear to you all of a sudden that there's an ambiguity, but actually there's none. Look at these two molecules carefully:
Note that the numbering for the molecule on the left will increase in the anticlockwise direction, while the numbering for the molecule on the right will increase in the clockwise direction. The pi bond in each case gets the number $1$ (and it should, because both the molecules are identical)
You seem to be assuming that the numbering for the molecule on the left also applies on the molecule on the right. But, that is not the case, as shown above.