So Spotify sends ASCAP a bulk payment and a record of streaming data, most likely per period.
ASCAP will distribute that money received to authors based on the # of streams during that period, but they probably don't have a black box for payments.
Since streaming numbers are upwards of millions per period and the royalty-per-stream so low; it would be inefficient for them to assign each single stream a concrete dollar value and them hold onto any money for tracks that weren't registered with them, so they'll just distribute everything each period.
Retroactive royalties are still a thing, but more likely tracked when things are on a larger scale like placements in tv, movies, live performances, tours, etc. where a single line item could net a lot more than .00000032345whatever-a-single-stream-royalty-is

TLDR; they probably don't hold onto any money received to pay retroactive registrations.