The solution to this is supposed to be in the changelog for the new version, but some internal issues have prevented us from updating our docs site. But the solution is easy: If targeting a device that has mender client 2.1 or earlier, with a mender-artifact tool 3.3 or newer, use the --no-checksum-provide flag when creating the artifact. In Yocto you can add this argument to the MENDER_ARTIFACT_EXTRA_ARGS variable.
When all clients are upgraded you can remove the argument again.