HA HBDev interface are not shown by checkused/cmdb refcnt show

v5.6.2 @ FGT60D:

FGT60D-2 # show system ha
config system ha
    set group-name "HA-ap"
    set mode a-p
    set password ENC fk...Q==
    set hbdev "internal7" 50 "internal6" 50
    set override disable
    set monitor "dmz" "internal5"
end

FGT60D-2 # diagnose sys cmdb refcnt show system.interface.name dmz
entry used by complex system.ha:monitor
entry used by table router.static:seq-num '1'

FGT60D-2 # diagnose sys cmdb refcnt show system.interface.name internal5
entry used by complex system.ha:monitor
entry used by table system.dhcp.server:id '1'

FGT60D-2 # diagnose sys cmdb refcnt show system.interface.name internal6
FGT60D-2 # diagnose sys cmdb refcnt show system.interface.name internal7
FGT60D-2 #

Why is FortiOS missing usage for HBDev interfaces? Bug?