Data syncing across devices but not with Atlas?

I have a to-do list on android which has been using Atlas as the backend for about a year now.
I recently ran into an issue where most changes on one android device are reflected on other logged in device but not in the atlas cluster. And apparently this has been happening since October of last year without me catching it, because my atlas cluster is not showing any new data since October.

Also back in October was when I upgraded from a shared cluster to a non-shared cluster. It would make sense that my clients prior to October will require a client reset, but should new clients be able to sync with issue?

Any guidance would be greatly appreciated.