Aha! Figured it out. Delete the loading icon and it all works perfectly, even the older project (Thunkable) from Unable to parse Web API response - #23 by bibbi.
Here’s that exact same project with the loading icon deleted and the loading icon blocks removed:
https://x.thunkable.com/copy/e523e49630f02d699a504dbfca4225af
The blocks don’t affect it, though, just the presence of the loading icon component.
This could be related to this bug report: Adding loading icon bugs project · Issue #864 · thunkable/thunkable-issues · GitHub