Recently updated a couple of FreeBSD hosts to fresh 13.0-RELEASE.
All hosts have NET-SNMP reinstalled from pkg after OS upgrade, version 5.9_3,1.
But values of UCD-SNMP-MIB::dskUsed and UCD-SNMP-MIB::dskAvail are completely freezed.
Even snmpd daemon restart, or remove+install makes no changes to data given via SNMP.
Only host reboot helps.
https://github.com/net-snmp/net-snmp/commit/93ea661906fb25657a63cfd757ef7a04c4dd0b33 makes net-snmp great again in 13.0)