I can confirm this issue. Thanks for the succinct bug report.

I can distil it down to something even smaller (mostly writing this down for my own memory for when I delve into fixing this):

mkdir -p /tmp/1944 && cd /tmp/1944 && php -n -d zend_extension=xdebug -dxdebug.mode=develop,profile,trace -dxdebug.client_host=localhost -dxdebug.client_port=9003 -dxdebug.log=/tmp/1944.log -dxdebug.output_dir=/tmp/1944 -r 'echo strlen("BUG");'

This bug is now fixed in GIT, but not exactly in the way you originally described.

What should have happened: no trace file, but only a profile file

The xdebug.start_with_request setting is by default "default". For the profiler, that means "yes", which means that the profiler will start at the start of the request, but for the tracer this default means "trigger" - i.e., only if XDEBUG_TRIGGER=<some value>.

Now that the bug is fixed, you can start the tracer by doing either (of course you can make these settings in php.ini):

XDEBUG_TRIGGER=start php -dxdebug.mode=tracer yourscript.php
php -dxdebug.mode=tracer -dxdebug.start_with_request=yes yourscript.php

cheers,
Derick

This should be documented in https://xdebug.org/docs/upgrade_guide#changed-xdebug.auto_trace , as the advised options are not enough.

UPDATE: The issue was on my side, ignore the comment above.