When a process definition has more than 1 version, and a version other than version 1 is running, then drilling down from the dashboard view to the Process Instance view by clicking on the Process Definition invokes the Process Instance view using version 1 as the URL parameter, resulting in no Process Instances to be shown (since version 1 is not running). A workaround is to manually select the version number in the filter, but that is annoying. Is this a defect or am I missing something? This is on Camunda 8.9 using a trial cluster.
This is a known open defect in Operate where the dashboard incorrectly defaults to version 1 in the URL when only a newer version has active instances. I found the following relevant resources:
- GitHub Issue #51617 - Operate dashboard does not query process instances of all versions when only one version has active instances
- GitHub Issue #51398 - Operate - Process Instance Version Selection
- Related forum post - Operate showing no process instances
Does this help? If not, can anyone from the community jump in? 
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You are not missing anything, this appears to be a defect in Operate 8.9. The dashboard drill-down selects version 1 instead of the version containing the active instances. Manually changing the version filter is currently the workaround.
Alternatively, c8volt provides a version-aware overview:
c8volt get process-definition -b C89_SimpleUserTask --stat
2251799814912481 <default> C89_SimpleUserTask v148/v2.0.0 [ac:8 cp:- cx:- inc:-]
2251799814912256 <default> C89_SimpleUserTask v146/v2.0.0 [ac:- cp:- cx:- inc:-]
2251799814912199 <default> C89_SimpleUserTask v145/v2.0.0 [ac:4 cp:- cx:- inc:-]
found: 3
This shows that versions 148 and 145 have active instances, while version 146 does not.