I've found bug fixing to be quite educative. It helps show all the things that go wrong.

Each time you solve a bug, you should think about what should have been done architecturally to prevent this problem or make it easier to find. This'll help you write better code in the 10% of time.

Of course if 90% of everyone's time is spent fixing bugs, that could be a sign that the code quality is low, and you might be missing out that way. But it could also be that the product is just "finished", feature-wise.

It's the same situation that I'm in now, and it makes me uncomfortable. I am maintaining a react app. I can't say it's because it allows you to learn new things, but it's surely a slower growth than designing and building entire apps from the ground up.