Undefined behaviors -- yes. But being able to trigger undefined behavior is not a huge foot gun by itself. Starting with good code examples means you are much less likely to trigger it.

Having a good, logical description of supported features, with a warning that if you do unsupported stuff things may break, is much more important than trying to define every possible action in a predictable way.

The latter approach often leads to explosion of spec volume and gives way more opportunities for writing bad code: predictable in execution, but instead with problems in design and logic which are harder to understand, maintain and fix. My 2c.