I have found the issue. I am marking this as a solution in case others come across this issues.
So, if you make the name of the CNAME www, please ADD www,domain,com to the custom domain and change it to the NEW target.
If your CNAME name is backup1,domain,com, then go to your glitch project and ADD THAT SUBDOMAIN as it, so like add backup1,domain,com, and update the target for that.
Ignore the ,'s, as I am only allowed to put 2 per reply.