I am rather curious too.
But, people in the stream file world often do a lot of deletes. For
may get purged automatically. You may not want to restore these. However,
I can see where not keeping track of this stuff shouldn't be a default.
I'll withhold further comment pending outcome of your pmr.
Another thought, if you truly do a FULL system save, and that file was
deleted before the full system save then the BRMS recovery report
shouldn't include any previous save. It just shouldn't restore it. But
if you did a wrklnkbrm it should retain the older volumes until those
volumes are expired and reused.
Does it think your library save was incremental, but your stream file save
was full? In that case, I can see it acting like what you've experienced.