I’ve updated my Mikrotik (rb751g’s and rb1200) devices to version 6.19 and since then my monitoring server could not monitor via SNMP. It seems snmpv3 bulk walk does not work. SNMP (v1) without bulk walking does work.
From 6.18 → 6.19. Had not noticed SNMP problems before.
Thanks for the link, i did found another old thread more specific about bulk walks, but not this one. It seems others also have SNMP problems with some recent firmwares.