  We present high-resolution dayside thermal emission observations of the
exoplanet KELT-20b/MASCARA-2b using the MAROON-X spectrograph. Applying the
cross-correlation method with both empirical and theoretical masks and a
retrieval analysis, we confirm previous detections of Fe\,\textsc{i} emission
lines and we detect Ni\,\textsc{i} for the first time in the planet (at
4.7$\sigma$ confidence). We do not see evidence for additional species in the
MAROON-X data, including notably predicted thermal inversion agents TiO and VO,
their atomic constituents Ti\,\textsc{i} and V\,\textsc{i}, and previously
claimed species Fe\,\textsc{ii} and Cr\,\textsc{i}. We also perform a joint
retrieval with existing \textit{Hubble Space Telescope}/WFC3 spectroscopy and
\textit{Spitzer}/IRAC photometry. This allows us to place bounded constraints
on the abundances of Fe\,\textsc{i}, H$_2$O, and CO, and to place a stringent
upper limit on the TiO abundance. The results are consistent with KELT-20b
having a solar to slightly super-solar composition atmosphere in terms of the
bulk metal enrichment, and the carbon-to-oxygen and iron-to-oxygen ratios.
However, the TiO volume mixing ratio upper limit (10$^{-7.6}$ at 99\%
confidence) is inconsistent with this picture, which, along with the
non-detection of Ti\,\textsc{i}, points to sequestration of Ti species,
possibly due to nightside condensation. The lack of TiO but the presence of a
large H$_2$O emission feature in the WFC3 data is challenging to reconcile
within the context of 1D self-consistent, radiative-convective models.
