No, I think it is normal. If it were easy to gain a mental model of the code simply by reading, then debugging would be trivial. The whole point of debugging is that there are differences between your mental model of the code and what the code is actually doing, that sometimes can't be uncovered unless you step through it line by line even if you're the one who wrote it.

It is why I am a bit puzzled by the people who use an LLM to generate code in anything other than a "tightly scoped" fashion (boilerplate, throwaway code, standalone script, single file, or at the function level). I'm not sure how that makes your job later on any easier if you have even a worse mental model of the code because you didn't even write it. And debugging is almost usually more tedious than writing code, so you've traded off the fun/easy part for a more difficult one. Seems like a faustian deal.