I'm having the same issue. Trying to get a distinct count of the CAR_ID field (string data type) in an indicator pane. Weirdly it works correctly when I use the distinct count of the CAR_ID field in the serial chart pane.
I also have this and a couple other layers aggregated as a FeatureSet using a custom data expression. This incorrectly shows the total value, not the distinct value when using the indicator pane (the distinct toggle does nothing in this scenario). It also works correctly in the serial chart pane.
See here for the feature layer used (Open Data item in AGOL is referenced from our on-prem ArcGIS Enterprise)
See here for my test dashboard