This used to work and stopped. Tried it with a new app and button too. Whenever I click an open app button when the record is expanded, it seems that the window message with the record id is not delivered to the app. I get the following error in the console:
Failed to execute 'postMessage' on 'DOMWindow': The target origin provided ('https://block--t-qtk-d2-z-an-c-dz-kv--b3r294g.alt.airtableblocks.com') does not match the recipient window's origin ('https://airtable.com').
The app DOES open in the extensions dashboard when clicking an app open button from the grid view.