It sounds like it is time for you to update your computer science knowledge by learning about race conditions.
When a question/issue raises up, it's likely that more than one person will try to answer. This is not even a shared CPU environment: it's true concurrency. But humans tend to have personal ways of doing stuff: there's the verbose one, the one that uses only two fingers to write on the keyboard, the one that answers to the telephone... and in the end you have that many answers overlap each other, even if their times are quite different. It's like a race... a race condition :)
In this case, I'd also enhance your answer to the OP giving enough evidence of the need to use flock (and marking such a modification clearly, in order to keep the following answers "meaningful"): the fact that your personal script used flock was not entirely evident :)
Don't fool yourself.
perl -ple'$_=reverse' <<<ti.xittelop@oivalf