Purging appears to completely empty the draftcndb, so that if you restart the server the first/lastChangeNumber values are zeroed.
The attached script can reproduce the issue - edit the few variables at the top to suite and run. Key points:
- before the purge, first/lastchangenumber, cn=changelog and the draftcndb look OK.
- after the purge, first/lastchangenumber are now both 4 (OK), cn=changelog is empty, but the draftcndb is empty.
- after the restart, first/lastchangenumber are 0.