If you're asking an LLM to compute something "off the top of its head", you're using it wrong. Ask it to write the code to perform the computation and it'll do better.
Same with asking a person to solve something in their head vs. giving them an editor and a random python interpreter, or whatever it is normal people use to solve problems.
base64 specifically is something that the original GPT-4.0 could decode reliably all by itself.
the decent models will (mostly) decide when they need to write code for problem solving themselves.
either way a reply with a bogus answer is the fault of the provider and model, not the question-asker -- if we all need to carry lexicons around to remember how to ask the black box a question we may as well just learn a programming language outright.