Oh my word. I might know!
I think that you need to add a pluginId
<script type="text/discourse-plugin" version="0.1">
api.modifyClass('model:user', {
pluginId: 'my-plugin',
testProperty: function() {
return 1;
}
});
</script>
It’s required on later versions, I actually submitted PRs for a couple themes yesterday to add this. I might be turning into a plugin/theme developer at long last