After some investigation (see thread below), this problem seems to be limited to RTX 3080 and 3070 and Linux. It can be worked around by disabling tensor cores. Restart MATLAB and run
setenv NVIDIA_TF32_OVERRIDE 0
before you do anything else. Further investigations are under way to look for a solution that doesn't require this workaround, which will reduce performance.
Are you running MATLAB release R2021a? The 3070 is not supported on earlier releases.