My guess is that you are using an ARM64 mac and that you are affected by Issues with pseudotube rendering on Apple M1 macs. If that is the case, please use the workaround detailed in the link above.
If you are not sure whether you are using an ARM64 mac or the above did not solve your problem, please provide the output of the following commands (copy-paste the full block into the terminal and press ‘enter’):
sysctl hw.model
sw_vers
mrview -version
cat /usr/local/bin/mrview
That should give us some insight into the hardware and software versions you are using.