Hi Brash, the "On" osnap is so generic it is intentionally not combined with other object snap labels if a "specific point" one is active.

That's because otherwise you would be seeing it added to every single label except "Cen", because they are all points on the curve.

So since that would be a lot of extra labeling that would be nearly always present, "On" is only displayed when there is not any more specific snap engaged.

- Michael