Why is echo not working in Doppler but printenv works fine?

I ran into this same issue and this is what I understood from mucking around with the environment. Doppler injects the secrets when the command is executed. That means that the containing shell process does not have access to the command. printenv is a command that uses the argument TEST_PASSWORD as a key within its own runtime. It doesn't depend on the underlying shell to resolve TEST_PASSWORD to the value you've mapped in doppler. However, when you use echo $TEST_PASSWORD, this won't have any value (or it will have the value that your encompassing shell may have already had, if it did), since doppler hasn't yet injected the value.

Try the same thing but instead, put your echo command(s) into a shell script and execute it with doppler run -- bash script.sh, it will work now because the encompassing shell then executes this command and doppler injects the values therein.