Found the issue that when initial sync is interrupted by a restart (or task rebalance), and after restart it doesn't redo the initial sync and start with reading oplog.

After some investigation, it turns out the root cause is during initial sync, it'll commit offset with initsync:true, but when reading the offset during connector creation, it just ignores that flag and takes this offset as valid ones in the code below:
https://github.com/debezium/debezium/blob/master/debezium-connector-mongodb/src/main/java/io/debezium/connector/mongodb/SourceInfo.java#L295:LL304

I created a pull request to fix it: https://github.com/debezium/debezium/pull/527