Ah, it was because I was expecting a parameter in the onComplete method - I see the onComplete now, along with these logs:
onConsoleMessage: Android Webview event sent
[INFO:CONSOLE(2)] “Android Webview event sent”, source: https://bigcartel.feathery.io/static/js/2.da45068e.chunk.js (2)
Thanks so much for helping me track this down!