  Pair-instability (PI) is expected to open a gap in the mass spectrum of black
holes (BHs) between $\approx{}40-65$ M$_\odot$ and $\approx{}120$ M$_\odot$.
The existence of the mass gap is currently being challenged by the detection of
GW190521, with a primary component mass of $85^{+21}_{-14}$ M$_{\odot}$. Here,
we investigate the main uncertainties on the PI mass gap: the
$^{12}$C($\alpha$, $\gamma$)$^{16}$O reaction rate and the H-rich envelope
collapse. With the standard $^{12}$C($\alpha$, $\gamma$)$^{16}$O rate, the
lower edge of the mass gap can be 70 M$_\odot$ if we allow for the collapse of
the residual H-rich envelope at metallicity $Z\leq{}0.0003$. Adopting the
uncertainties given by the STARLIB database, for models computed with the
$^{12}$C($\alpha$, $\gamma$)$^{16}$O rate $-1\, \sigma$, we find that the PI
mass gap ranges between $\approx{}80$ M$_\odot$ and $\approx{}150$ M$_\odot$.
Stars with $M_{\rm ZAMS}>110$ M$_\odot$ may experience a deep dredge-up episode
during the core helium-burning phase, that extracts matter from the core
enriching the envelope. As a consequence of the He-core mass reduction, a star
with $M_{\rm ZAMS} =160$ M$_\odot$ may avoid the PI and produce a BH of 150
M$_\odot$. In the $-2\,{}\sigma{}$ case, the PI mass gap ranges from 92
M$_\odot$ to 110 M$_\odot$. Finally, in models computed with $^{12}$C($\alpha$,
$\gamma$)$^{16}$O $-3\,{}\sigma{}$, the mass gap is completely removed by the
dredge-up effect. The onset of this dredge-up is particularly sensitive to the
assumed model for convection and mixing. The combined effect of H-rich envelope
collapse and low $^{12}$C($\alpha$, $\gamma$)$^{16}$O rate can lead to the
formation of BHs with masses consistent with the primary component of GW190521.
