Ref.: https://area51.phpbb.com/phpBB/viewtopic.php?p=333941#p333941

If there is an error in the ext.php, this extension can not be installed in 3.3, you will not get an error message.

Confirmed:

Note: I have debug enabled in the yaml file as per https://phpbbstudio.com/viewtopic.php?f=11&t=130

I tested with an extension ready for 3.3 and I put a wrong square bracket there on purpose.

I was able to see the debug error only with F12 (developer tools)