That is a very good point.
The IMBALANCE......MONITOR ecam action in the SHUT DOWN procedure is quite ambiguous.
It is just a monitor advice, not a prompt to carry out the inmbalance procedure as such. Besides, it is not required to carry out this procedure if there isn't a fuel imbalance advisory.
I had this advisory once. The captain decided not to carry out the procedure before the advisory came for that note in the procedure even though we had detected quite an important umbalance. After an hour, we had a permanent bank angle and some sideslip. Then the advisory was triggered.
So it is left to the pilot to decide on wether to open the xfeed valve or keep it shut.
Therefore, in the double tank low fuel caution we should shut off the XFEED unless the MONITORing of the fuel umbalance suggests opening it before an engine flames out. In case of a previous eng fail, I guess the sensible thing to do is open the valve.