Querying for the system for supported SpeechRecognizer Languages using this method I've noticed that on Android 8 (Oreo) the returned results are empty. I've tested this on a Google Pixel and a Google Pixel C both running Android 8. It all works on all devices available to me running an Android version lower than 8. I couldn't find any changes to the way this is supposed to be done on Android 8.
Coming back to answer my own question. Set the package of the Intent to "com.google.android.googlequicksearchbox" to receive the results for Android > 8.
A fix was discovered in this answer here.
The actual issue (setPackage requirement) is discussed in this question.
An issue has been filed to Google's Issue Tracker here.