Update: Migrating the simplest possible Rails app from free-tier to free-tier didn't work on Render (deploys fail, no logs to be found), so I'm bailing out to try Fly.
To any render people reading this, it appears to have been an issue with the migration plugin and the Heroku-18 stack. Manually deploying the app worked fine.
Thanks for the heads up and good to hear you were able to deploy the app manually. We might deprecate the migrator because it's hard to keep up with all the Heroku stack and buildpack configs and because manual migrations are relatively straightforward.
To any render people reading this, it appears to have been an issue with the migration plugin and the Heroku-18 stack. Manually deploying the app worked fine.