Canon EOS R3 Advanced User's Manual page 366

Caution
HDR movie recording is not available with [ : IS (Image Stabilizer) mode] set to
[ Digital IS], or when set to [ : Highlight tone priority], [ : Canon Log
settings], or [ : HDR shooting ].
Since multiple frames are merged to create an HDR movie, certain parts of the
movie may look distorted. This is more noticeable in handheld recording affected
by camera shake, so consider using a tripod. Note that even if a tripod is used for
recording, afterimages or noise may become more noticeable, compared to normal
playback, when the HDR movie is played back frame-by-frame or in slow motion.
Image color and brightness may change significantly for a moment if you change
settings for HDR movie recording. Also, the movie will not be updated for a
moment, and the frame stops momentarily. Be aware of this when recording
movies to external devices via HDMI.
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