I guess it's possible that if someone hasn't updated their device in a long time, this could be a problem, but yeah... this particular emoji is not a new introduction so I'm not sure what's going on.
If you're saying that the test email comes through with it looking OK and that the final one doesn't on the same device, then it's possible something's happening on Squarespace's end to break the Unicode. Or that certain providers are blocking certain things.
Re: spam - you may want to reconsider using emojis altogether. It's interesting how things come full circle and there are many who now automatically think spam when there are any emojis in the title because people have adapted to it being a marketer trick (I definitely fall in this camp - if I see an email with an emoji in the title, I automatically delete it without opening). I've also read that some email providers are starting to look for emojis as an indicator of spam, too.
Here's an article by constant contact, for instance that advises to only use a single emoji if you're going to use one so as to not trigger spam filters: https://knowledgebase.constantcontact.com/articles/KnowledgeBase/17990-using-emoji-in-an-email-subject-line?lang=en_US