Good.
This question highlights to us the difference between management and leadership. A good manager, rightly focusing on effective structures, processes and facts, is not necessarily a good leader.
To be effective, a manager should also be a good leader. Different styles are necessary for different people, as you imply. The leadership style must somehow be adjusted to the context. The goal is always to achieve maximum performance, and the leadership must be tuned to that according to the team you have, how long they have been working together with you and experience with the job at hand.
It is clear, from the description of the manager’s behaviour, that the manager has a task-oriented leadership style (micro-managing).
The Task-oriented manager focuses on designing effective structures and processes to guide and motivate people. This type of manager wants "just the facts" and tends to be viewed as rigid or controlling.
This style is, indeed, effective with new employees because they need information and feedback., i.e. they require close supervision, attention and advice. While, the experienced employees do not need such a close supervision and guidance, information and feedback. That is why the new staff members work effectively, but within three months, four of the more experienced training professionals quit. While the other two spend very little time working and seldom fill out their weekly reports.
Employees must deal with new bosses and their new style. However, it is the boss’ responsibility to get results through the resources he/she has entrusted.
The new manager did not understand that he needed to adopt different leadership style for different categories of employees with the aim of getting the best performance out of them all. Nor he did manage the change and communication adequately. This created resistance and low morale.