No it is not, if you ever tried to write portable C code you should clearly be aware that each C compiler has its own flavor of undefined behavior.
The C standard is very generous in terms of undefined behavior freedom.
The C standard is very generous in terms of undefined behavior freedom.
If you're using some other definition, then say so, otherwise your comments make no sense.
They are free to do that and still be compliant, after all UB means anything goes.