InfinitePool maintains a list of all objects that have been created and not yet removed. For SFSBs this essentially functions as a second cache and makes passivation pointless, since passivating from the SFSB cache doesn't remove the ref in the InfinitePool.

IIRC, the intent in 5.x is to get rid of the use of pools for SFSBs, but this is broken in 4.x as well.