I'm reminded of a book I read once called "No More Mr Nice Guy" where it explains that "nice guys" are actually kind of jerks because they behave certain ways to try to manipulate people and they expect reciprocation.
They live by a sort of unspoken code that they believe exists but which other people may or may not be aware of.
When a "nice guy" doesn't get his way, he can become bitter and resentful.
(I am using the term "guy" to fit the name, but it could be a woman too)
I'm not sure if you intended to describe your manager as "nice" in this way, but niceness has become a huge red flag to me ever since reading that book. I much prefer someone who is good (in competence and/or in behavior) than someone who is nice.