Problem/Motivation
node_help() does this:
<?php
$type = $node->getType();
return (!empty($type->help) ? Xss::filterAdmin($type->help) : '');
?>$node->getType() returns the name of the node type, as a string. The next line assumes $type is a NodeType object. As a result, no help text is displayed.
Note that the default node types (article and page) don't have any help text by default, so to make the problem visible you have to add one (on admin/structure/types/manage/article)
Proposed resolution
Do NodeType::load($type) or so.
Also, does it make sense to test the presence of the help text? I cannot find any test like that in Core...
Remaining tasks
Fix and perhaps add test coverage.
User interface changes
The help text will be visible again :D
API changes
None