I was able to successfully get the Google Lens app to work in multi-app Kiosk mode on an android tablet by:
-Assigning the "Google" app com.google.android.googlequicksearchbox to the kiosk device group (even though Google is already installed on the device as a native app).
-Adding the Google app to the custom layout along with the Google Lens app in the device configuration profile.
-Clearing the cache and all data in app settings for the Google app.
It appears Lens is actually dependent on the Google App as Lens would just show a black screen before completing the extra steps.