In my experience, it's been the assumptions about knowledge gained from having a degree in computer science. Many developers who hold this degree have assured me they didn't learn anything practical / have forgotten everything they learned. I've then heard the same devs crack jokes with each other about linked lists, and they also seem to instinctively know how libraries written by others were most likely built.
I think that some folks with a degree in computer science take for granted the small snippets of wisdom that help them do their job well every day. It's easy to forget where you learned something, or why a certain concept is straightforward to you at first glance.
For many years of my career, not knowing a lot about data structures and common algorithmic patterns definitely affected how fast I learned new concepts, and had a negative impact on quality of the architectural patterns I designed. When I mirrored other patterns I saw, I often didn't understand why it was a good pattern. Of course, not everyone would experience this, but personally this was true for me. I know many self taught folks who never had these learning problems like I did (perhaps they're a lot brighter than me).
I am self taught and over the past couple of years have filled in some of the CS learnings I missed out on. It was like a light bulb moment for me, where everything had so much more clarity than it had in the past. Additionally EVERYTHING WAS SO INTERESTING TO LEARN OMG. I LOVE hashtable theory, convex hull solutions, runtime vs memory usage, bloom filters etc. etc.
As a self taught programmer, I still struggle with feeling like I belong in this industry, but it's never too late to go back if you know you have gaps. It's made me love computers even more.