If you provide the full XOG write and just change that one piece, does it work as expected then?
Not all XOG files are allowed to just be cut down to minimal data changes for submitting.
Furthermore, even those that do work (intentionally or not) this way, when you provide some data, there can be an expectation that other data also accompanies it (i.e. if you change a start date without also giving a finish date, then the finish date may end up being changed in a way that is undesired/unexpected).
So in this case, if you XOG out a project, retain all the existing allocations data, make the change to the availTo attribute and XOG it back in, does that make a difference compared to your earlier results with the minimal attempt?
#helpinghands