Middle management is a really easy area for a growing company to end up weak in. If you promote a junior manager (out of necessity, or as a reward for being a very good junior manager) who's never seen what good higher level management looks like, they don't even know enough about what they don't know.

And then most junior managers never end up seeing it. Took me a while before I did - I liked my boss, even, but then they left and someone new came in, and the difference was night and day. Being able to come into a new place and figure out what details to focus on and what to not, identify common problem areas, identify people to delegate to and trust, get upper-management support for you when necessary, figure out where you're still uncomfortable and how to get you to start stretching yourself there... it's not stuff I knew I was missing. And I could've gone another decade without knowing it, probably!

Late edit: the more I see of the skills it takes to be good at it, and how easily it can be a thankless role, the more I'm kinda terrified even more of that jump than the dev->manager jump.