I guess it's possible that there is some additional security policy in the way. I would try either using /tmp/xdebug.org to see if that works, and otherwise I would like to see an "strace log" from the specific worker. The easiest way to create such a thing is possibly to reconfigure FPM to only have one worker process, and then attach strace to the non-main FPM process with:

sudo strace -o /tmp/strace.log -e open -p <pid>

You can find the pid by using "ps aux | grep fpm". If there are two processes listed, it is likely the one with the lowest PID.

After strace has started, make a request to trigger the problem. Then stop strace (with Ctrl-C) and attach the /tmp/strace.log file, which should have an entry about "xdebug_log" in it.

I could not reproduce this, and I did not get more information when requested. Hence I am closing this ticket. If you can provide more information, feel free to reopen it.