We set deployment.custom_domain_check_enabled to false on all our applications but they started doing the domain checks again:
💥 Resolution of CNAME for domain XXX.dev failed. Please check that you have correctly configured your CNAME. If you are using a CDN you can forget this message
I think the flag was broken by your update here. We fixed the issue by also setting the new flag to true. It seems the old advanced settings flag is no longer relevant?