The person who stays pleasant under pressure may be postponing the cost until they get home.

Composure in difficult hours is not the same as being unaffected. Holding a steady voice takes effort, and effort has to be paid for somewhere, usually later and usually in private. The people around them see calm and conclude that it was easy, which is how the bill ends up invisible to everyone except the person carrying it.

If someone you know handles hard days smoothly, ask how the evenings are rather than how the day went. If you are the one holding it together, consider leaving a little room after work before anything else is asked of you.