I found out that the problem with multiDisk table starts after the monitoring systems runs a discovery of installed hotfixes.
The discovery consists on 3 bulk requests to the following OIDs:
1.3.6.1.4.1.2620.1.6.20.10.1.2
1.3.6.1.4.1.2620.1.6.20.10.1.5
1.3.6.1.4.1.2620.1.6.20.10.1.4
After querying those, the multiDisk table stops working.
Very weird behaviour, but working with TAC they were able to confirm it and create a hotfix. They said it was in queue to be included in the JHF, fix number PRHF-34122, but still no sign of it.
You can restart SNMP by running 'set snmp agent off' and 'set snmp agent on' in clish, or disabling and enabling it in the Gaia WebUI.