What the cause of '.�{�t�.' does not meet criteria'?

Problem: A SNMP monitor checks a MAC address in a router. The config is:

<service name="StpDesignatedRoot" interval="300000" user-defined="true" status="on">
   <parameter key="retry" value="5"/>
   <parameter key="timeout" value="3000"/>
   <parameter key="port" value="161"/>
   <parameter key="hex" value="true"/>
   <parameter key="oid" value="1.3.6.1.2.1.17.2.5.0"/>
   <parameter key="operator" value="="/>
   <parameter key="operand" value="20 00 c0 7b bc 74 98 00"/>
</service>

It worked for many months. Yesterday it failed with reason Observed value '2000c07bbc749800' does not meet criteria '= 20 00 C0 7B BC 74 98 00'. For us, it looked like the format has been changed. Maybe an update on the router. But we didn’t update OpenNMS or the router. So nothing obviously has been changed.

We’ve enhanced the monitor to fetch also this and some other states like:

<service name="StpDesignatedRoot" interval="300000" user-defined="true" status="on">
   <parameter key="retry" value="5"/>
   <parameter key="timeout" value="3000"/>
   <parameter key="port" value="161"/>
   <parameter key="hex" value="true"/>
   <parameter key="oid" value="1.3.6.1.2.1.17.2.5.0"/>
   <parameter key="operator" value="~"/>
   <parameter key="operand" value="2000C07BBC749800|20 00 C0 7B BC 74 98 00|2000c07bbc749800|20 00 c0 7b bc 74 98 00"/>
</service>

And it worked fine again.

Until today…

The event reason now is: Observed value ' .�{�t�.' does not meet criteria '~ 2000C07BBC749800|20 00 C0 7B BC 74 98 00|2000c07bbc749800|20 00 c0 7b bc 74 98 00'
And it also resolves automatically after some cycles.

Expected outcome: Maybe not complaining about the observed values? Or giving me hints why it happens.

OpenNMS version: 28.0.0

Two SNMP monitor issues in one week. Maybe there is a relation to What's the cause of `cannot convert 1.3.6.1.4.1.8072.3.2.10 to a HexString`?. But as I said, we didn’t update the components.