On my production Jenkins system, Workflow jobs which run a shell script that happens to print Unicode characters instead display the null/question mark placeholder in the console log view. The same shell script, run by a freestyle job, prints correctly. I can't reproduce this on my test Jenkins server, so I suspect this is partly an environment issue, but I don't know what to check to confirm that and it's interesting that it only impacts Workflow jobs.
Workflow script which reproduces this:
sh 'env printf "\u2024 \u2024 \u2024 \u2024 \u2024 \n"'