The 18th and the 22nd are PointSavers rates.

When booking a reservation on points with 5+ nights, all PointSavers are removed because you cannot double dip with PointsSavers and fifth night free.

Try booking 4 nights while including the 18th or the 22nd, you'll see that PointSavers is included in the rate again (you can see this by clicking Customize Payment under the room).

It is not a gimmick, it is just the nature of 5th night free - it cannot be combined with other offers (free night awards, cash and points, PointSavers, etc).