Versions:
Scaler 2.7.2
Ableton Live 11.2.7
macOS Monterey on M2
Unlike VST3, the AU version of Scaler does not provide an input as shown in this screenshot. Which makes the AU unable to send notes to another track, and prevents this routing Use Case.
This is a limitation of the AU spec and not Scaler. Scaler cannot add what is not in the plug-in specifications. Until Apple changes that you need to use the VST or VST3 version. This works perfectly in Ableton Live.
Cheers and welcome to the forum.