The conditions where mdb and hdb/bdb need dumping/reloading are usually going to be different. It would be nice if we would avoid dumping and reloading databases that don't actually need it. A proper fix for #759596 will probably get us most of the way there.
Beyond the unnecessary (and failing:) dump/reload, I also got a gratis DB_CONFIG file during upgrade from wheezy to 2.4.40-1. So not only the conditions differ, but also the methods, since MDB does not need this.
Thanks; confirmed. The comment in the maintainer script says: # If there is an old DB_CONFIG file, restore it before # running slapadd which is fine, but then it also restores the default DB_CONFIG if there wasn't an old one. That's probably not necessary, since the olcDbConfig bits already specify the default DB configuration.