Currently, there is a check that prevents "Age" from being set to less than zero:
however, when messing around with Vanilla falling_block entities, the traditional way of preventing the block from despawning is to set the life of the falling_block to negative max int (- 2147483647) using the summon command (summon falling_block ~ {Time:-2147483648}). With this check here, we are unable to set the ticks lived of a block using spigot and instead need to do it through vanilla mechanics instead. Please consider removing this check.