I guess I don't view completing an item as a formatting change. It's an action on my list, a to-do that I'm completing. Making text bold is a formatting change. To me it's like being in a contacts app, and saying I want to stay in a contact when I deleted it.
Plus, recall, this is with the setting on that already says I want to Hide completed items. Anyway, that's why I mentioned in the linked issue that this could be a preference.