I was using an extension that adds a "Create New Topic" button in forum index.
And on the index there is no forum associated. So when clicked, the ext go to:
posting.php?mode=post, then it picks that and shows a special page, similar to the default new topic page with 1 extra field: Forum selection on an dropdown.

I can't bypass the problem with posting.php?mode=post&f=0 because it doesnt pass on the if validation and throws same error. Only way to bypass is to use f=1 and make a little change on the extension. But doesn't feel correct.

The ext is using the only event that can be used:
'core.modify_posting_parameters'    => 'modify_posting',

But of course, with this change this feature is not possible anymore. Since we're thrown with this:
https://github.com/phpbb/phpbb/pull/5760/files#diff-2bd852c3b9a70bfaabb479ef5b67b657056c1e74de09e433e52a644092ec28c9R60

Probably better to see from here the culprit line:
https://github.com/phpbb/phpbb/blob/master/phpBB/posting.php#L60