Phil57. Surprised by your comment that assignments are made based on trees as well as segments. The original statement said it was wholly dependent on shared segments. Do you know if they have changed the way they make these assignments?
Sorry, yes I've just checked and they are now saying that the assignments are being made solely from DNA, although originally they were utilising an algorithm to refine parental sides via trees. Probably, as they've increased the database and refined their processes, they've removed that aspect.
But they can't phase your DNA. Most autosomal tests including Ancestry's are unphased, which is partly the reason that lower length matches are less accurate. They are calculating the phasing of base pairs by comparison between matches who share the same string lengths. As the size of the database and number of comparisons increases, the accuracy should also increase, but the ultimate results are still governed by how they calculate and produce them.
https://support.ancestry.co.uk/s/article/SideView-TechnologyI still suspect an issue with the latest update. As I say, my both sides allocations are all at low match lengths (apart from my brother) where the probability of IBC matches is relatively high.