Not at all. It would be surprising if it wasn't maintained.
Evolution can progress very rapidly, but the core machinery which drives life is very conservative.
A good analogy would be to look at other codes. In the computer world, consider ASCII. Each number maps to control code, letter, number or symbol. This encoding is entrenched. Imagine how hard it would be to change a single letter of ASCII to mean something else. Most of the hardware and software on the planet would require updating. It's not just that interoperability is important. It's that every piece of software on a single computer system would require updating, in lockstep, to transition from the old to the new encoding. This would be an almost impossible feat.
The same constraint applies to DNA encoding of protein (and other) sequences. There are multiple pieces of the machinery which would require changing in synchrony for the result to work and result in a viable living organism. A triplet coding system change would require almost all instances of that coding triplet changing to retain existing structure and function in every protein using it, new enzymes to synthesise the new amino acid and tRNA, along with all of the associated regulatory and control systems. Only at that point could you start using the new amino acid triplet sequence in a new or modified protein. Evolution works by single small changes and natural selection. Making several big changes is extraordinarily unlikely.
It's easier to make such a fundamental change in simpler organisms where the scope of the change is limited. And this is likely why the small number of variations we see in genetic encodings are both in the lowest forms of life, and are largely superficial. The current encoding is entrenched as a result.
