Could there be patterns in ciphers? Sure. But modern cryptography is designed specifically against this. Specifically, it's designed against there being patterns like the one you said. Modern cryptographic algos that are considered good all have the Avalanche effect baked in as a basic design requirement:

https://en.m.wikipedia.org/wiki/Avalanche_effect

Basically, using the same encryption key if you change one character in the input text, the cipher will be completely different . That doesn't mean there couldn't possibly be patterns like the one you described, but it makes it very unlikely.

More to your point, given the number of people playing with LLMs these days, I doubt LLMs have any special ability to find whatever minute, intentionally obfuscated patterns may exist. We would have heard about it by now. Or...maybe we just don't know about it. But I think the odds are really low .