NetOne
Not looked at the c3p as I'm not using C3 yet, but from what you've written you're spawning a new Pickup every second at the same location as the old pickup (which is the same time you wait to flash), so it could be hiding the one that is flashing behind it. You might want to extend the wait to spawn to 3 seconds instead of 1.
Also, rather than count dt in a variable, you could use the Timer behaviour on the Pickup
Pickup: On created -> Pickup: Start Timer "StartFlash" for 1.0 (once)
Pickup: On timer "StartFlash" -> Pickup: Flash 0.1 on 0.1 off for 1.0 seconds
Pickup: On flash ended -> Pickup destroy
Edit: Here's a C2 capx example - https://www.dropbox.com/s/huyfxdvwxtttu ... .capx?dl=1
I spawn to the left every second, to the right every 3 seconds, notice that the one to the left doesn't appear to flash (because new one appears in front) even though all the other code is affecting them the same with regards to flash and destroy.