Sharing problem in To-Do

Hi! Any advice on resolving this sharing problem on To-Do?

I have successfully shared a list with a collaborator, in the sense that, once the share is accepted, both of our names appear as sharing the list.

However, when either of us adds items to the list, they appear on our devices alone - not on our partner's. If notifications are on, we get notified of changes to the list, but without the changes being inscribed in the list.

Anyone encounter this problem? Any advice?

We've tried all sharing methods, creating new lists, etc.