[I've verified this bug still exists in LR 10.0. See this post. -- John Ellis]
After a while of custom sorting the order of photos and stacks within a collection (using Grid View), Lightroom starts to unpredictably refuse to sort photos. Some will get positioned where I drop them, others won't move at all and yet others will get positioned somewhere close by (eg. 4 or 5 photos before or after).
After digging into the catalog file I've come across what I think is the problem, but don't know how to fix it. In the attached file you'll see a screenshot of the database table for the collection, you'll see I've hilited the images that are part of the same collection, but their positionId is identical (which should never happen I'm assuming), probably due to the field size reaching it's maximum length. This is what I believe is causing the problem. Tested this on both Lightroom 5.7 and CC 2015.8.
This is a major bug and effectively stops user sorting from being functional, as well as now having potentially lost weeks of work. Any suggestions?