It looks like pendingDocumentIDs only updates its value if the replication has been started (and hasn’t stopped yet.) It should still work if it’s in the offline state, though.
If you’re using a non-continuous replication, then if you’re offline it’ll stop almost immediately and you’ll have this problem. You could use a continuous replication as a workaround.
I’ve filed a bug report because we should make this property work reliably whether or not the replication is running.