Again, when it's yellow, it isn't a locked target, it's a memory track of previous/ lost target.
So you can lock a contact, get dat about it (track & speed), break lock and you will get velocity vector for that target. As long as the contact keeps constant track and speed, the green echo will stay with yellow ghost contact.
At the same time, you can get another lock on another target.
So you will get one radar track (green) + one memory track (yellow) on screen.
Like said, you can erase yellow track with theta up, and you can prevent it from re-appearing by un-selecting "RDO" on PCA's top row while in AA mode.
RDO is ON by default.