The only reason I have my job that I do now is because of side projects. How else are you suppose to learn different parts of the stack when you're (usually) only hired to work in one part? How are you suppose to learn DevOps/CloudOps unless you're on an Ops team? Many (most?) companies don't delegate full stack responsibilities to individual teams.
A learning project is a project you're doing with the express intent of learning a new technology. The focus is on the learning, not the delivered functionality.
A side project is a project you're doing for the community. It's a labor of love. The focus is on functionality and meeting the needs of your users, not learning technology. You may even be able to grow it into an actual business.
From a career perspective you want to focus on learning projects. As you've mentioned learning projects are very important for your continued learning. Side projects on the other hand have the risk of distracting you from your work and my advice would be to avoid them unless you're scratching your own itch or maybe even want to explore starting a side business.
Bottom line - be clear with yourself on what you're doing and why you're doing it.