Those are compliance errors. That is, if the spec allows it (as "undefined behavior") then it's not a bug. If the spec doesn't allow it, then there is a defined behavior and it is a bug.
For example, a hardware bug is one that does not comply with the specification. Either the spec must be changed (perhaps allowing the existing behavior), or the bug fixed.
I suspect that is where we differ. It seems to me that if a C program does something that results in undefined behavior according to the C spec, it is undefined behavior period, regardless of any other factors, because the spec says it's undefined and that's all that matters.
I'm saying that when the Python language specification prescribes a behavior, and the Python implementation in C has a different behavior because it does something which the C language specification considers undefined, then the Python implementation in C does not comply with the Python language specification.
I'm saying that it's incorrect to say that non-compliant behavior with respect to the Python specification is the same thing as undefined behavior. It makes no sense to say something is undefined when the specification defines what is supposed to happen.
Pretty much every language has some undefined behavior. That undefined behavior can be invoked intentionally, or because of bugs.
C has a lot of undefined behaviors. Even simple addition can result in an undefined behavior because int overflow is undefined. Any C program that takes numbers as input (via the shell, or FFI, or whatever) and adds them together can exhibit undefined behavior. Null pointer dereferences are also undefined, and pretty much any C program of reasonable complexity can encounter those, in the form of bugs.
A bug in hardware can also cause these undefined behaviors to be invoked -- for example, a friend of mine who does embedded programming had some null pointer dereferences or something like that because the hardware did not set a value when he told it to set the value. Null pointer dereferences are undefined, and if they happen because of hardware bugs, that is a case of hardware causing undefined behaviors to happen. It would not be reasonable to say they were not undefined behavior simply because they were caused by hardware instead of programmer error. The fact that it happened at all is what matters, not what caused it.
Even if a language does not have any undefined behaviors whatsoever, that's not the end of it. If a program in that language interacts with other programs that do have undefined behavior, such as C programs, it could potentially trigger or be affected by undefined behaviors in those other programs. If that happens, it's not reasonable to say that it was not undefined behavior simply because it happened in a piece of code in a different language. (So, if the result of running a Python program is affected by undefined behavior in the C implementation of Python, that program has undefined behavior -- even if the Python code does not. The undefined behavior is C's, not Python's -- but it happens because of running the Python code, which caused the C code to run.)
In other words, any invocation of code that causes undefined behavior to happen -- whether it is intentional, or caused by a software bug, or even caused by a hardware bug -- counts.