Basically our index is only persisted in shared memory, not disk. When deleting we only remove the entry from the index. If you reboot the machine or the process crashes the shared memory cannot be reused and the index is rebuilt from disk (in community edition the index is always rebuilt from disk). So deletes that were not overwritten on disk will return.
As for the records not deleting there has been some work around to detect anomalies that may cause this. We haven’t been able to reproduce these locally, if this is still happening in your environment if you can reproduce with the latest build we can walk you through using these tools.