Will the most current Essential work with Moodle 3.1.2?
Yes. Are there issues you're having?
No, just trying to avoid issues! I really like the theme, and a while back following an update my Essential theme disappeared. In case you can't tell, I am pretty new to Moodle. I was a bit panicked, since I thought everything was gone for good, but as soon as the new Essential was released and I installed it, everything was back. . I did not want to update to 3.1.2 if it meant I would loose my Essential for a period of time.
The version for M3.1.x is still at release candidate status. There are 'issues' with it and versions for other Moodle releases: https://github.com/gjb2048/moodle-theme_essential/issues.