When I use a dispenser to dispense bone meal on a potato field, the bone meal grows the potatoes at a slower rate than when I use the bonemeal manually.

Whenever I use bonemeal on a potato plant, it grows the plant to full size (ready to harvest).

To reproduce:

Sow potato on farmland. Use bonemeal manually. Observe how it grows to full size. Replant potato, place dispenser beside potato plant, put bonemeal in dispenser and activate with a redstone signal. Observe that the potato grows but only partially.

Expected result: Potato grows regardless of how bonemeal is used.

Observed result: Potato growth depends on whether the bonemeal is used manually or in dispenser.