  The gravitational-wave (GW) detection of GW190521 has provided new insights
on the mass distribution of black holes and new constraints for astrophysical
formation channels. With independent claims of GW190521 having significant
pre-merger eccentricity, we investigate what this implies for GW190521-like
binaries that form dynamically. The Laser Interferometer Space Antenna (LISA)
will also be sensitive to GW190521-like binaries if they are circular from an
isolated formation channel. We show, however, that GW190521-like binaries that
form dynamically may skip the LISA band entirely. To this end, we simulate
GW190521 analogues that dynamically form via post-Newtonian binary-single
scattering. From these scattering experiments, we find that GW190521-like
binaries may enter the LIGO-Virgo band with significant eccentricity as
suggested by recent studies, though well below an eccentricity of $e_{\rm 10Hz}
\lesssim 0.7$. Eccentric GW190521-like binaries further motivate the
astrophysical science case for a decihertz GW observatory, such as the
kilometer-scale version of the Midband Atomic Gravitational-wave
Interferometric Sensor (MAGIS). Pre-merger observations of GW190521-like
binaries with such a decihertz GW detector would be able to constrain the
eccentricity of GW190521-like binaries to greater precision than with just
LIGO-Virgo alone. These eccentricity constraints would also provide additional
insights into the possible environments that GW190521-like binaries form in.
