[Babul]

iam afraid it has something to do with the coordinates. 3760 is the east ocean at los santos, its not too far away form the regular map, but i bet, R* coded a limit for pickup coordinates. some users encountered similar problems where pickups disappear when you go away and back to them again - like in the cityhall, its in the sky, out of boundaries, you create a pickup, its fine, go up to the 2nd floor, go back to the pickup at the entrance hall, it wont show up again.
if nobody else finds a better solution, i have to tell you: try to spawn the pickup as far away from the beach as long as its not bugging out, then you will need to modify your map...