That is not a secondary HUD reticle.
That symbol is called the Ghost Velocity Vector marker (GVV).
The aircraft Velocity Vector marker (VV) is caged laterally on all HUD modes, except AG. The GVV appears to show the lateral velocity vector component of the VV. This lateral velocity vector can be due by winds, maneuvering, etc.
In the previous version, it wasn't working correctly and only appeared when the lateral velocity component was very high.
In AG mode, the VV is uncaged so you can use it to correctly aim at ground targets.