Canon EOS-1D X Mark III Advanced User's Manual page 753

Caution
Images captured with auto rotation set to [Off] will not rotate during playback even
if you later set auto rotation to [On].
Note
If a picture is taken while the camera is aimed up or down, automatic rotation to the
proper orientation for viewing may not be performed correctly.
If images are not rotated automatically on a computer, try using EOS software.
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