Bottom-Tier should still be Bottom Tier. If the lowest Tier in the match is a Tier 9, it should qualify as a Bottom-Tier match.
I've heard folks say they've charted out 20 games and found inconsistencies with the % values listed for Top/Bottom Tier stuff, so there might be a Time-in-Queue override, though if there is I don't have any information regarding it. In general, it should aim for the target values of Bottom/Top assuming there's enough players to works with and avoid Queue Dump weirdness.