Value for "Mark messages read after … seconds" cannot be changed

In the Preferences, I changed this from 0 seconds to 10 seconds.

Later, I wanted to change it to two seconds. However, even though I change it and close the Preferences window, it remains at 10 seconds.

I even quit Mailmate and restarted. It made no difference.

What I have now realised is that if the insertion point is still in the field, then it does not save the changed value. Only if I click in another field, does it save the value.

This does not happen with the font size field, which does remember the change even if the insertion point is still in the field.