CloudKit subscriptions, sharing, and remote notifications

I am hoping you can tell me what's possibly different here.

Devices A and B are shared participants in a shopping list.

I've confirmed:

• both are on the Production environment

• both have database subscriptions to private and shared

What I'm seeing:

• A (share owner) sees items added by B.

• B (invitee) does not see items added by A. If B syncs then it sees the items, but it receives no related remote notifications at all, despite the subscriptions (yet A does receive them)

Seems like these two devices should be the same, but what might be causing B to not get the remote notifications?

Thanks for any pointers.