My best guess is that this bug is caused by an edit on a build actually being an "old" build being replaced by a "new" build server-side. When I shoot the builds the damage is dealt to the "old" build (before it was edited) but doesn't reach the server until after it's already been deleted and replaced by the "new" (edited) build.
Another issue caused by this bug is if a player edits open a build where your shot would not hit the build during the timeframe the invincibility bug applies, even if your crosshair is directly over the opponents face they will take no damage and your shot will blank. It totally negates damage done by your shots.
This bug can be shown in a more dramatic fashion in this fast editor's video. He is able to prevent a wooden cone that is at 1hp from breaking through AR spray.