  By probing the population of binary black hole (BBH) mergers detected by
LIGO-Virgo, we can infer properties about the underlying black hole formation
channels. A mechanism known as pair-instability (PI) supernova is expected to
prevent the formation of black holes from stellar collapse with mass greater
than $\sim 40-65\,M_\odot$ and less than $\sim 120\,M_\odot$. Any BBH merger
detected by LIGO-Virgo with a component black hole in this gap, known as the PI
mass gap, likely originated from an alternative formation channel. Here, we
firmly establish GW190521 as an outlier to the stellar-mass BBH population if
the PI mass gap begins at or below $65\, M_{\odot}$. In addition, for a PI
lower boundary of $40-50\, M_{\odot}$, we find it unlikely that the remaining
distribution of detected BBH events, excluding GW190521, is consistent with the
stellar-mass population.
