When use Q-encoded to handle non-ASCII characters, if raw characters start with non-ASCII, we will get encoded like '=?UTF-8?Q?=XX=XX?='. IOS(>26) Mail App may think the first '?=' end of the '=?UTF-8?Q?=' is then end flag, so show incorrectly.
Mail filename of attachment show incorrectly
Are you interacting with Mail via some API? Or are you hitting this while working with the Mail app as a normal user might?
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Quinn “The Eskimo!” @ Developer Technical Support @ Apple
let myEmail = "eskimo" + "1" + "@" + "apple.com"