Indeed this situation would rise only under misusage of the log, where multiple processes inherit by  mistake or by wrong design a file descriptor of a same log file (Regarding the threads case I unfortunately cannot reproduce).
While this kind of misusage doesn't have terrible consequences when not using the rotator, the growing used disk space which occurs when using the rotator is more serious, that's why I found appropriate to try and protect the programmer from his own mistake. But I understand if this is out of scope.