  We analyze the prompt emission of GRB 100724B and GRB 160509A, two of the
brightest Gamma-Ray Bursts (GRBs) observed by Fermi at $\lesssim{\rm MeV}$
energies but surprisingly faint at $\gtrsim100\;$MeV energies. Time-resolved
spectroscopy reveals a sharp high-energy cutoff at energies $E_c\sim20-60\;$MeV
for GRB~100724B and $E_c\sim80-150\;$MeV for GRB~160509A. We first characterize
phenomenologically the cutoff and its time evolution. We then fit the data to
two models where the high-energy cutoff arises from intrinsic opacity to pair
production within the source ($\tau_{\gamma\gamma}$): (i) a Band spectrum with
$\tau_{\gamma\gamma}$ from the internal-shocks motivated model of Granot et al.
(2008), and (ii) the photospheric model of Gill & Thompson (2014). Alternative
explanations for the cutoff, such as an intrinsic cutoff in the emitting
electron energy distribution, appear to be less natural. Both models provide a
good fit to the data with very reasonable physical parameters, providing a
direct estimate of bulk Lorentz factors in the range $\Gamma\sim 100-400$, on
the lower end of what is generally observed in Fermi GRBs. Surprisingly, their
lower cutoff energies $E_c$ compared to other Fermi-LAT GRBs arise not
predominantly from the lower Lorentz factors, but also at a comparable level
from differences in variability time, luminosity, and high-energy photon index.
Finally, particularly low $E_c$ values may prevent detection by Fermi-LAT, thus
introducing a bias in the Fermi-LAT GRB sample against GRBs with low Lorentz
factors or variability times.
