I posted on the fly.io forums informing them of the issue with custom domains and glitch. They deployed a workaround but said that the code that glitch is using is outdated and using deprecated features. I’m assuming this fix is only temporary, so glitch probably needs to update their custom domain code to use non-deprecated features.
Thank you for posting this! Could we get @glitch_support to take a look here, please?
And by the way, I have moved this to #feedback.