  Gravitational waves provide a window to probe general relativity (GR) under
extreme conditions. The recent observations of GW190412 and GW190814 are unique
high-mass-ratio mergers that enable the observation of gravitational-wave
harmonics beyond the dominant $(\ell, m) = (2, 2)$ mode. Using these events, we
search for physics beyond GR by allowing the source parameters measured from
the sub-dominant harmonics to deviate from that of the dominant mode. All
results are consistent with GR. We constrain the chirp mass as measured by the
$(\ell, m) = (3, 3)$ mode to be within $0_{-3}^{+5}\%$ of the dominant mode
when we allow both the masses and spins of the sub-dominant modes to deviate.
If we allow only the mass parameters to deviate, we constrain the chirp mass of
the $(3, 3)$ mode to be within $\pm1\%$ of the expected value from GR.
