I’m seemingly not having trouble with any other emoji, however when I use the puzzle emoji - 
- when i save and return to the subject line, this character is getting converted into a ? (question mark).
Any idea why this is happening and how to overcome this issue?
Yes, I have faced the same issues, both in the headline & in the body section as well, can’t use any emoji
I would love to see this feature added
Have you tried other emojis?
The map emoji - 
- seems to work without any issues (as far as I can see) so you can get some to appear in the subject line, but others will not strangely.
OMG What a fun topic! Thank you soo much for asking this.
After giving it a duckduckgo, I found this:
It even shows right away in Gmail 
Gave it another spin:
Hey Joey, it seems the linkjis no more accessible.
Oh… anyeay now you can just directly copy emojoes to the subject line. Since Mautic 3+ it should work as the subject is saved with the proper encoding.
Cool. I’m will give that a try.