I eventually figured this out with some help from this thread, where a user suggested appending 2> /config/command.log to the end of the ssh command in order to write the output from the command to a file. Doing so yielded this error:
Identity file id_rsa not accessible: No such file or directory.
I just had to update the path to the id_rsa file to include /config/, and everything works as it did before. So, @VDRainer was correct. Even though I wasn’t calling a script, I was still referring to the location of id_rsa, so the exact same issue applied. Thank you for your help! Also, just for reference, the config now looks like this:
command_on: "ssh [email protected] -i /config/id_rsa -o StrictHostKeyChecking=no 'echo on 0 | cec-client -s -d 1 && echo as | cec-client -s -d 1'"