Having first hand knowledge about the state of the codebase is helpful to doing your job as a tech lead / manager. Doing bug fixes / small features is a productive way to gain and maintain this knowledge.
There is absolutely no need to do a (potentially junior) engineer's job. It might be 'productive' but it's not the best use of your time or your role, and thus it is in fact a waste of time and money.
These 'junior engineer' tasks are attractive for a manager who still wants to be in the code. They are non-critical and small. Perfect for someone with an unpredictable schedule. However, they also provide an opportunity for the manager to have a better, but still imperfect sense of the codebase and technology.