I marked this as Major as it results in updates with the Changed Files package not being executed as intended (the /vendor not being replaced by a new /vendor.

See https://www.phpbb.com/support/documents.php?mode=install&version=3.2#update_all

also

https://www.phpbb.com/support/documents.php?mode=install&version=3.3#update_all

also install.html in the download packages

The point is that the changed files version x to x+1 doesn't mention that the old /vendor has to be deleted and the new /vendor uploaded. This has resulted in a couple of failed updates.