I left the game running for several hours, leaving my colony to do its thing. Then I discovered... why is this dupe not freaking out even at max stress? He's been like this for several minutes as of posting this bug report. He probably got so used to it, ugly crying becomes a waste of time.
1 - Stress out a dupe and make sure conditions remain stressful. For better results, get all skills from the skill tree.
2 - Play or leave the colony to do its thing for several hours, preferably upwards of 6 hours. Eventually the stressed out dupe will not do any stress reaction, even at 100% stress.