This may be a programming choice rather than a bug per se but it annoys me so...
PC/XP:
In the clip view, copy (Ctrl+C) a midi note that is longer than the grid resolution (ex: an 8th note on a 16th note grid). Paste (Ctrl+V) the copied note. Live will drop the new copy into the clip by moving the copied note over one grid unit (in this case a 16th note) overlapping the 8th note original. Move your new copy where you'd like it. No problem you now have two 8th notes in the clip. Now, 'undo' this move. Move the copy again. The original just got truncated to a 16th note as the overlap from the 'undo' was deleted.