Apparently, UDisks behaves strangely in these two ways:
if you try to get a UDisks.Object, it often fails. We solved this with a retry loop.
if you make UDisks enumerate all possible objects, it could fail to list the recently-created ones. Here we solved the problem by eliminating the need for it. The UI was doing this just after creating the partition, so that the new partition would pop up among the other UDisks objects. Just getting the specific object we need leads to easier refactoring.