  When exposed to high magnetic fields, certain materials manifest an exotic
superconducting (SC) phase that attracts considerable attention. A proposed
explanation of the origin of the high-field phase is the
Fulde-Ferrel-Larkin-Ovchinnikov (FFLO) state. This state is characterized by
inhomogeneous superconductivity, where the Cooper pairs have finite
center-of-mass momenta. Recently, the high-field phase has been observed in
FeSe, and it was deemed to originate from the FFLO state. Here, we synthesized
FeSe single crystals with different levels of disorders. The level of disorder
is expressed by the ratio of the mean free path to the coherence length and
ranges between 35 and 1.2. The upper critical field $B_{\rm{c}2}$ was
systematically studied over a wide range of temperatures, which went as low as
$\sim$ 0.5 K, and magnetic fields, which went up to $\sim$ 38 T along the $c$
axis and in the $ab$ plane. In the high-field region parallel to the $ab$
plane, an unusual SC phase was confirmed in all the crystals, and the phase was
found to be robust to disorders. This result suggests that the high-filed SC
state in FeSe may not be a FFLO state, which should be sensitive to disorders.
