I'm not sure if this is a bug. Maybe something I've missed

However, I've seen old extensions for eZpublish using myextension/settings/debug.ini.append.php, the same way as I do. So I think an extension should be able to override debug.ini.

However this is not the case for me...

Steps to reproduce

In an extension, I've created the file settings/debug.ini.append.php containing:
[GeneralCondition]
dmv-workflow=enabled

And in a template operator created by this extension (for example), I do:
eZDebug::writeError("my test string with eZDebug", "my test label with eZDebug" );
eZDebugSetting::writeError( 'dmv-workflow', "my test string with eZDebugSetting", "my test label with eZDebugSetting" );

I can see the first log, but not the second one...

Afterwards, I've put my debug.ini.append.php directly in settings/override (so not in an extension but in the override settings directory of eZpublish), and at this moment, the 2 following lines work as expected. I can see 2 lines in the error.log

eZDebug::writeError("my test string with eZDebug", "my test label with eZDebug" );
eZDebugSetting::writeError( 'dmv-workflow', "my test string with eZDebugSetting", "my test label with eZDebugSetting" );

So as a conclusion, it seems eZpublish doesn't override the debug.ini with the debug.ini.append.php from extensions