Variable substitution for environment variables does not work when using the command line argument --remote-env

I tried to extend the PATH environment variable used inside the container from the command line like this:

devcontainer exec --remote-env PATH='${containerEnv:PATH}':/my-additional-path --workspace-folder . bash

However, it seems the variable substitution does not work here. When I add the same to the devcontainer.json, then the variable substitution works as expected.

Am I doing something wrong, or is it just not implemented? If not, would it be possible to add this feature?

Thanks,
Thomas