Solved
How to solve character encoding issues in subject with emoji and special characters?
We're trying to send out an email with a fire emoji, represented by =?UTF-8?Q?=F0=9F=94=A5?=
It works fine for the most part, until an email contains higher ascii such as © or ó, at which point it starts trying to mix encodings and set the rest of the title as windows-1252 according to the header output, at which point you see the above =?UTF-8?...etc string printed out in the subject.
We're importing the data as UTF-8, and even tried the "Force Encoding" setting in the master template, but it still generates a broken, mixed-encoding title when it hits higher ascii.