I’m using To-Do on Chrome (62.0.3202.94) on macOS. When I add an item, it sometimes doesn't appear until some other action takes place -- e.g., adding another item, or refreshing. See the attached video.
I would suggest posting the issue here: https://todo.uservoice.com/ as that's where the product group will engage with it.
Or do a service request and pass that file on to the support team.
Please navigate to the To-Do settings page (found under the O365 settings cog icon) and click "Copy Session ID", which is the last option in the settings. Then contact us via https://todosupport.helpshift.com/a/microsoft-to-do/?p=web&contact=1 with that Session ID so we can see what's happening.