I was able to work with Google Cloud Support in order to get a response on this issue. The resume functionality of the Firestore gRPC Listen call does not support delete capture.

In order to determine if there was a delete, you can include the expected count and then compare the last known value to the new one upon resuming. Additionally, you would need to count the number of new documents and increment your stored expected count as needed. If there is a difference between that and the new value coming from the server upon resume, that means deletes have occurred. If you wanted to know what was removed, you'd need to get all the changes again. This is not particularly helpful if your source regularly has deleted documents, but this is the intended functionality.

The resume functionality does fully support adding new documents, updating existing fields on documents, and removing fields from documents.