A manager who doesn't continue to code or work on the day-to-day actions may fail to keep up with necessary technical knowledge, effectively reducing their ability to manage.
The problem with a boss coding is that they don't dedicate enough time to management. Connections, management theory and functional knowledge are important.
Instead of a boss who is part-time manager, have we tried managing 3/4 of the year then doing a "developer rotation"? How does that work?