Why use a sharded cluster with a single shard? Why not just a single replica set? Is it to be future proof because you are planning to scale or something like that?
As you have a single replica set, can you run the following command in your shard so we can have an idea of the health status of your oplog?
test:PRIMARY> rs.printReplicationInfo()
configured oplog size: 990MB
log length start to end: 29251secs (8.13hrs)
oplog first event time: Thu Sep 17 2020 12:30:26 GMT+0000 (UTC)
oplog last event time: Thu Sep 17 2020 20:37:57 GMT+0000 (UTC)
now: Thu Sep 17 2020 20:38:00 GMT+0000 (UTC)
I suspect this is happening because you went through an election and your client is trying to restart the Change Stream but cannot because the previous known point has already been overwritten in the oplog because it’s too small.
Cheers,
Maxime.