We did more tests, seems that there is no special :’ comparator in the Shopify API, what happened is that Shopify treated [email removed] the same as a@b.com (ignoring the malformed “email:” prefix).
For anyone that is struggling with searching for non-Latin emails (e.g. [email removed] in Shopify, here are our test results:
a@b.com -- returns too many results
"a@b.com" -- returns the correct result
email:a@b.com -- returns 0 results
email:'a@b.com -- returns too many results
email:'a@b.com' -- returns 0 results
email:"a@b.com" -- returns 0 results