Steps to reproduce:
1. Start play and then press record MIDI, so left edge of MIDI item not start on grid division.
2. Quantize recorded notes 100% by grid.
3. Zoom in maximum to left edge of any quantized note.
4. See that note start is not on grid, a bit earlier or later.
5. Snap left edge of item to grid and glue.
6. Now see on notes like in p.4 - they now start on grid.