Late packets are “too early” because the microsecond counter that triggers them rolls over every 72 minutes, hence one that is slightly too late is 72 minutes early for the time the hardware would try to transmit it - which the software prevents.
Ultimately though it’s worth realizing that while interesting this issue of the network holding onto packets until only a second remains before they are due is moot on a network with a 1 second RX1 - At most, pushing the packets earlier might make OTAA join work. ADR and all ordinary downlinks would still FAIL