The box marked optional can still feel compulsory to someone afraid of being overlooked.

Optional means one thing to a secure person and another to someone who suspects they are already on thin ice. If attendance, extra hours, or the additional form are voluntary, the people most likely to opt out are the ones who feel safe, and the people who cannot risk being seen to decline will do all of it. What you have created is a quiet sorting mechanism rather than a choice.

If you offer something optional, say plainly what happens to people who skip it, and mean it. Then check who is turning up, because that tells you whether the word optional is doing any real work.