I think this problem is caused by the long "detection time", it takes 6+ jumps for me to loose detection of some POIs, and because I always want to go to the (new) one with highest thereat, then the low stuff gets accumulated, and I can only visit 1 system per jump, that means I can't jump to all of them just to clear out the POI from the detection, so they expire just before (or after) I must leave the zone.
Apparently it needs some way to increase the detection range (see more POI) without increasing the detection duration (amount of jumps before expiring.