CName randomly giving 521 error

I currently have a site set up with an ‘A Record’ subdomain pointing to my backend IP (api.domain1.com). I have a few other sites pointing to that A Record. I try to point another domain (domain2.com) to the first site and it returns a 521 error. I have multiple domains pointed to api.domain1.com and only some of them are returning the 512 error although all of them are configured the same way (eg. domain3.com returns the desired content)

Any thoughts on why? This might be a really confusing question.
Thanks for any help

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