  The nearby GRB221009A at redshift $z=0.1505$ has been observed up to a
maximum energy of 18 TeV with the LHAASO air shower array. The expected optical
depth for a photon with energy $E_\gamma=18$ TeV varies between 9.4 and 27.1
according to existing models of the extra-galactic background light (EBL) in
the relevant mid infra-red range. The resulting suppression of the flux in
several (but not all) EBL models makes it for these EBL models unlikely that
this photon could have been observed at the claimed energy. If the photon event
and its energy are confirmed and possibly even more photons above 10 TeV have
been observed, the photon-pair production process would have to be suppressed
by mechanisms predicted in extensions of the Standard Model of particle
physics. We consider the possibilities of photon mixing with a light
pseudo-scalar (e.g., axion-like particles; ALPs) in the magnetic field of the
host galaxy and the Milky Way and Lorentz invariance violation (LIV). In the
case of photon-ALP mixing, the boost factor would reach values $\sim10^6$ for
photon couplings not ruled out by the CAST experiment, but limited by other
astrophysical constraints. Viable scenarios would require either very efficient
mixing in or near to the GRB or that the largest part of the total luminosity
is radiated at TeV energies, different from previous GRB afterglows. In the
case of LIV, required boost factors are achievable for a LIV breaking energy
scale $\lesssim 2\times 10^{29}$~eV ($\lesssim 4\times 10^{21}$~eV) for the
linear (quadratic) modification of the dispersion relation. A more simple
explanation would be a misidentification of a charged cosmic-ray air shower.
