This doesn't look like a bug, but instead is the result of the printer not actually reporting accurate toner values via SNMP.
From the Printer-MIB:
prtMarkerSuppliesMaxCapacity
Registration: 1.3.6.1.2.1.43.11.1.1.8
Access: read-write
Reference: None
Description:
The maximum capacity of this supply container/receptacle
expressed in SupplyUnit. If this supply
container/receptacle can reliably sense this value, the
value is sensed by the printer and is read-only;
otherwise, the value may be written (by a Remote Contol
Panel or a Management Application). The value (-1) means
other and specifically indicates that the sub-unit places
no restrictions on this parameter. The value (-2) means
unknown.
prtMarkerSuppliesLevel
Registration: 1.3.6.1.2.1.43.11.1.1.9
Access: read-write
Reference: None
Description:
The current level if this supply is a container; the
remaining space if this supply is a receptacle. If this
supply container/receptacle can reliably sense this
value, the value is sensed by the printer and is
read-only; otherwise, the value may be written (by a
Remote Contol Panel or a Management Application). The
value (-1) means other and specifically indicates that
the sub-unit places no restrictions on this parameter.
The value (-2) means unknown. A value of (-3) means that the
printer knows that there is some supply/remaining space,
respectively.
So if the printer only reports whether there is some toner left, but not how much, it returns -3. It also will return a -2 for the capacity.
The code for calculating the toner takes the current level (in this case -3), divides it by the capacity (-2), and multiplies by 100. This will be 150 everytime, so long as there is any toner left. To make this work "correctly", the printer manufacturers would have to modify their code to report the specific values for the level and capacity via SNMP.
after running its nightly discovery our Observium now updated all the toner levels to Unknown ?
have you managed to have a look at the RicohPrivate MIB I have - http://anywhere.support/ricohmib