Yeah that's pretty common. If you'd like to do more machine learning, as your team and company grows you might try asking your boss to hire more SDEs so that you can spend more time with machine learning. Or alternatively, ask for more training so that the backend engineering goes more quickly.

As for "keeping up with the field", I don't recommend worrying about it. It's challenging, maybe impossible, to actually stay up to date on everything even if it's only ML. I find it's better to make a habit of learning something every day, however small, and focus on the growth aspect rather than some sense of "falling behind".