Ok, previously we had an known issue with towersof scaffolding sometimes leaving the most upper block hovering in the air, with the 3-drop issue you described. Didn't know it's possible in that shape pattern as well.
I currently got no clue what exactly is causing this bug, exspecially because it doesn't occur always (rough guess, with 3-towers it hapens ~70% of all trys).
On the other hand it's nor a gamebreaking bug. Scaffolds aren't exactly expensive, and the capability to duplicate them manually isn't much of a tragedy. Thus you won't see ahotfix for just this, though i hope we can find the exact cause up to the next update.