I think this is probably correct behavior. The IFD will start a turn within 2.5 miles of the waypoint, as a fly-by. If the turn angle is greater than 135 degrees, it would need to start the turn much earlier to not overshoot the outbound course. By overflying the waypoint before turning to the outbound course it is possible to stay in protected airspace. For example, at 150 Kts, the turn diameter at standard rate is only 1.6 Nm. Without GPSS, navigation would normally be fly-over, and this is no different. A direct turn to the outbound course would overshoot and require an S-turn correction. This method gets you on course closer to the waypoint. Garmin doesn't document what it does for the GTN series, though it is possible to select either fly-over or fly-by on a G1000.