serial_xfr_acquired time in the xfrd state file is not updated properly. This may cause an issue on restart if serial_xfr_acquired+expire < now. The zone is then not served (despite having had a recent incoming XFR and up to date SOA). After next XFR the zone will be served again.
Reported by Havard, [Opendnssec-user] "not serving soa" warning message